So, I think you can posses another humanoid body permanently through two castings of the Magic Jar Spell. Let’s see:

1 - Cast Magic Jar on a container;

2 - Possess an humanoid body;

3 - Cas Magic Jar again on another container;

4 - Repossess your body;

at this point:

a) the humanoid soul is in the first container;

b) your soul is in your original body;

c) the second container is empty;

c) the humanoid body is empty;

d) there are two castings of the spell coexisting.

moving on:

5 - Position the humanoid body more than 100 ft away from the first container;

6 - Destroy the first container, ending the first casting of Magic Jar;

at this point:

a) The humanoid souls can’t return to its body and he dies;

b) Your soul must return to your original body or you die;

c) Your soul is already is in your original body, you live.

d) The second casting is still active;

e) Humanoid body is empty;

f) The second container is empty.

moving on:

7 - Make sure the humanoid body is 100 ft or less from the second container;

8 - Destroy the second container, ending the second casting of the spell.

at this point:

a) your soul return to the humanoid body (original body of this casting);

b) Your original body is empty;

c) There is no active casting of Magic Jar.

So, you are permanently in your new body, without any casting of Magic Jar active. You have your original body as a spare body.

What do you think?

Nope.  Nystul's Magic Aura says "You change the way the target appears to (spells and magical effects) that detect creature types, such as a paladin's Divine Sense or the trigger of a symbol spell"   Parentheses added to clarify the spell.  

Divine Sense detects a creature type.  Symbol detects a creature type and activates.  

Magic Jar does not detect a creature type, it simply does not work except on a humanoid creature, anymore than Hold Person.  You can make an undead detect as a humanoid, but you can still Turn it while you could not Hold Person it.

By the Rules as Written, this would work.  There is nothing that would stop it, but it is a smells like a Rob the Super Munchkin way of doing things.  Only real way the DM could stop this would be saying the second casting does not work for Rules as Intended.  But as far as RaW go for it.

This is hilarious, could we get one more experienced player to triple check this xD

Quote from nightspirit2099>>

1 - Cast Magic Jar on a container;

2 - Possess an humanoid body;

3 - Cas Magic Jar again on another container;

4 - Repossess your body;

at this point:

a) the humanoid soul is in the first container;

b) your soul is in your original body;

c) the second container is empty;

c) the humanoid body is empty;

d) there are two castings of the spell coexisting.

There's a problem here: spells can't stack with themselves, so you can only have one Magic Jar effect active. Strictly speaking since the effects are equal in power, the most recent casting would win, releasing your original hostage and putting you back in your body at the same time that you're going into a new jar, and now your DM has a headache. In all likelihood they'll just rule the second casting does nothing to keep things simple.

even if they don’t stack, the second casting will not dispel the first. They will overlap.

Correct, it wouldn’t dispel your spell, it’s effects just ceases to work as you cast your other. The only way you may cast two spells in this way is by having one of them a concentration you are maintaining while you’re casting another non-concentration spell. 

Please correct me if I am wrong someone!

I think that wizards are very intelligent and clever, always investigating and experimenting with spells. So I think this is something that could fit well in the history. The DM can come up with consequences for this.

But, if when you cast the second Magic Jar the first casting ends, it’s even better. It means:

1) your soul go into the second container;

2) the first container is destroyed along with the soul that was inside;

3) your soul is forced to go back to your original body.

At this point:

a) Your soul is in your original body;

b) first container is destroyed;

c) the second container is empty;

d) the host body is empty.

From there:

1 - Destroy the second container;

2 - Your soul returns to the host body;

at this point:
a) You are in the new body;

b) There’s no Magic Jar active;

c) You have an catatonic spare body. 

Quote from Tyrannosary >>

Correct, it wouldn’t dispel your spell, it’s effects just ceases to work as you cast your other. The only way you may cast two spells in this way is by having one of them a concentration you are maintaining while you’re casting another non-concentration spell. 

Please correct me if I am wrong someone!

 You can have as many spell effects active as you want. Obviously the concentration rules limit you to one concentration spell at a time, and most non-concentration spells are designed to end if you recast them early. But if you want to cast Grease a bunch of times from atop your Phantom Steed while you have Light, Mage Armor, False Life, Longstrider, Darkvision, Mirror Image, Tenser's Floating Disk and Unseen Servant active, go right ahead.

The issue here is that you can't combine the effects of two castings of the same spell and Magic Jar has a range of Self, so it targets you. You can't have two active Magic Jar effects on yourself at the same time; one of the two has to be suppressed.

Combining Magical Effects

The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect--such as the highest bonus--from those castings applies while their durations overlap, or the most recent effect applies if the castings are equally potent and their durations overlap.

For example, if two clerics cast bless on the same target, that character gains the spell's benefit only once; he or she doesn't get to roll two bonus dice.

It's not entirely obvious how this should work for Magic Jar, but the logical way to suppress its effects is to send the souls back but let the container stay intact. That's what I'd do if my players started mixing Magic Jar an Antimagic Field.

Quote from nightspirit2099 >>

I think that wizards are very intelligent and clever, always investigating and experimenting with spells. So I think this is something that could fit well in the history. The DM can come up with consequences for this.

Sure, there's examples of spells going wrong in the books. "That time a wizard got their soul stuck in the wrong body" could be a fun thing to add to a D&D session, either as a bit of history or as the background for an NPC or player. I'm just saying that the rules are designed to prevent this so I wouldn't try to pull this as a player in the middle of a live game.

When you cast the Magic Jar a second time the first effect is suspended and you are under the effect of the second casting. What that means? That you will not be sent to your original body, because the first instance of Magic Jar will produce no effect over you. If you procede to destroy the first container, the first instance of the Magic Jar will still end but it has no effect over you, only over the soul that is inside. That’s why I’m still not convinced this method would not work. Thanks for the reply.

Make Simulacrum.

Cast Magic Jar.

Have Simulacrum cast Wish to make the effect of that Magic Jar spell unable to be dispelled by anyone except you.

Unless you have a dick DM, your Magic Jar now lasts indefinitely and without risk of it being dispelled.

Repeat process another day to make it immune to Antimagic Field.

(Why Simulacrum? These are non-basic uses of Wish and have 1/3 chance of losing ability to cast wish. By having your Sim use the wish, you don't have to worry about the risk.)

-

It requires a favorable DM but works RAW without worry over spell-stacking rules.

I think the container could still be broken and the spell ending that way. Also, I’m trying not to use any other spell.

The only thing I didn’t get is step 7. Also, I think that a creature that loses its soul through Magic Jar enters a catatonic state, which RAI don’t decay or need food or drink.

Awesome, thank you very much. Also I’ve been long wondering how D&D Beyond hasn’t made you a moderator yet!! Seriously though, excellent understanding of the game, great site interaction and help, etc.!

There's multiple places this can fail, depending on interpretation:

1. Possessing an inert body may not be possible, or if possible, may cause you to be comatose.
2. Having two magic jars simultaneously active may not be possible.
3. "Your body" is not defined. It's perfectly plausible to say that it's always your original body.

Quote from Brian_Avery >>

1. Since this spell is specifically designed to have your soul possess other bodies and have them under your control it is safe to assume you are not comatose. Also note that you are not possessing a comatose body, when you cast the second magic jar and go into the second container this causes the first spell to end and that causes you to return to your body.

2. Two magic jars are not simultaneously active, the casting of the second one will end the first, and that is how this is designed to work.

 In that case, it's irrelevant; the first jar ends, putting you back in your original body (or possibly killing you) and then, if you survive, you cast magic jar, putting your spirit in the jar, and your original body is your original body.

Quote from Brian_Avery >>

3. "Your body" is defined as your original body that you are in when you cast the spell, not any host body that you possess.

 "Your body" is not defined at all.

Quote from Pantagruel666 >>

Quote from Brian_Avery >>

1. Since this spell is specifically designed to have your soul possess other bodies and have them under your control it is safe to assume you are not comatose. Also note that you are not possessing a comatose body, when you cast the second magic jar and go into the second container this causes the first spell to end and that causes you to return to your body.

2. Two magic jars are not simultaneously active, the casting of the second one will end the first, and that is how this is designed to work.

 In that case, it's irrelevant; the first jar ends, putting you back in your original body (or possibly killing you) and then, if you survive, you cast magic jar, putting your spirit in the jar, and your original body is your original body.

Quote from Brian_Avery >>

3. "Your body" is defined as your original body that you are in when you cast the spell, not any host body that you possess.

 "Your body" is not defined at all.

 Nowhere in the spell description exits the expression “original body”. Your body it’s the body you currently possess and control.

I'd allow it, but don't think there won't be consequences for stealing a someone's body and just releasing their soul right into the welcoming arms of the DM.

If I was that soul, I'd want my body back...

Quote from Brian_Avery >>

Pantagruel666... you state that it is irrelevant, the first magic jar spell ends and I go back into my original body (possibly killing me) and then, if I survive, cast the second magic jar

why would the 1st magic jar end if I have not yet cast the second...

 Because two copies of the spell cannot exist simultaneously. To prevent that situation, the first spell must end before the second spell starts.