I have a character that I would like to use, but I have a problem with her. Our game exists on several different planes, what I want to do is have this character to be able to exist on at least two planes at the same time. Basically, I would like to have her exist on one plane (her home) but have a double exist on one of the other planes and be able to control that double from her home plane. Is this even possible? Our game is mostly HomeBrew, so there must be some way I could do this.

Oh, one more thing, if the double takes damage, that damage is transfered to the original as well. It would be as if she is on the other plane, but she is actually on her home plane.

This would be up to your DM. If the double dies does the original die? If there is no real in game benefit to having the original on one plane and double on another, then It’s basically just an RP thing. If the original is researching clues of the campaign while the double and the party just carries on like normal  then I could see that as a problem. Again, up to your DM if they would allow this. 

May I ask why you want to do this?

Actually, the purpose of the double is to keep the original out of harms way. Eventually the others will catch up with the original at some point, but for now she wants to stay hidden. I have been thinking about the damage thing, instead I thought it would be better if the original did not take the same damage as the double, and she won't die if the double dies. However, she is only able to create 1 double per day. Bassicaly, I want the double to take her place until she is ready to join the fight herself. There is a reason for this, I just can't explain everything because the game we are in is quite complex. I asked the DM, she told me that if I can make it possible using standard rules, then she is ok with it. I just can't figure out how exactly to do it. Would I have to homebrew the clone spell so that it works on separate planes. As far as I know, cloning only works if the copy and the original are on the same plane.Also, any information the double aquires (spelling?) , the original does learn, and the original knows where the double is at all times using a crystal ball to monitor the double's moves and actions. BTW, the original is a Warlock (witch) who must always perform a ritual to cast spells, however the copy would not have to do this, she would use spells in the normal fassion without rituals.

Yeah, I thought of that, I guess I could have her perform a ritual to modify the clone spell to work on separate planes. I would have to give some sort of limits though. Thanks for the input, I think I will go with that option. It seems the most plausible.

To do something equivalent to that using semi-official content, you'll need to be very high level.

If this is something you are allowed to do at low level in a homebrew game, then just get permission from the DM and don't worry about the mechanics.

The most straightforward solution would be a variation of Astral Projection, where you and your double are connect by a Silver Thread.

There is a reason doing this sort of thing with the standard rules is a high-level ability. It is a very powerful thing.

How are the other players going to feel about this? Not the characters -- the players. Your character can't die the same as theirs in a risky situation. Your character might be more likely to take chances that could affect the entire party, because although they perma-die if things go bad, your character doesn't.

As a DM, I would probably not allow this... I certainly would not make up special rules to allow you to do this, as it seems unfair to everyone else at the table, whose character has to have actual skin in the game from day 1, while your character is allowed to hide in another dimension and let a Simulacrum or an Astral Projection or what have you take all the hits.

If the double and the original both take damage at the same time and exist on different planes of existence, a mechanical solution to this is moot.  You're character is playing in a party, it's original is elsewhere.  Are there any mechanical advantages to playing a double with the original on another plane?  If not, this is just role playing flavor and needn't need any special status?

If anything, your character in the party you're playing in has a massive vulnerability the rest of the players don't.  If someone targets the original, your character in the party that's actually playing the game could die.  

EDIT:  Saw your update note where damage doesn't transfer. That's too much to ask.  Why should your character have such an advantage over the other characters in your group?  D&D is a team sport not an exhibition contest of who is the most special, at least in the best cases.

Your DM said you can do it if you use the standard rules.  Homebrewing is not the standard rules.  So if you're playing a high level game, make a character of suitable level that can Astral Project, but that spell can get both killed.  Other than that, Clone, but that means a vessel waits for you when you die.  If you try to homebrew something that gives you access to those powers at a lower level you're going against your DM's instruction and you're basically creating a cheat.

What you describe is used in AD&D for Gods.  The Gods exist on their own plane but they send avatars to other places.  You can't kill a God by killing the avatar, you have to go to their plane and kill the original.

Happy New Year!  Good luck and enjoy the game.

I think you could do most of this with an Echo Knight and a lot of roleplay. If you want weird homebrew mechanics about damage-sharing and not dying and stuff, that's getting into issues of unbalance within your party. But without changing any rules, you are totally free to say that if you die as an Echo Knight, your other self lives on in its world. You just don't get to play as them anymore.