Assume the following:

• A Sorcerer has the Find Familiar spell.
• The Sorcerer is engaged within range of touch of an enemy
• The Familiar is engaged within range of touch of a different enemy who is 15 feet away from the Sorcerer

 Can the Sorcerer attack both enemies by using Twinned Spell on a spell with a range of touch (such as Shocking Grasp) by using the Familiar's ability to deliver such spells for the Sorcerer?

I would make this an arcana check to determine whether or not the sorc could manipulate the weave in such a complicated way. Probably using their spellcasting ability instead of normal intelligence

Anmutu, I'm inclined to agree with you, but just curious if you know of an official rule the states it clearly?

Obviously I read those rules before asking the question, but both Find Familiar and the metamagic Twinned Spell are examples of spell/class features that change or bend previously established rules. 

The Casting a Spell section states "The target of a spell must be within the spell's range."  So when a Sorcerer casts a range of touch spell (without using Twinned Spell) through the Familiar, even though the Sorcerer isn't within melee range, technically the target enemy is in range due to the way Find Familiar changes the established range rules.  

Because of that I could see it being possible to use Twinned Spell and deliver the attacks as described in the question because for a single cast of a range of touch spell, both enemies would technically be within the spells range.

Like I said, I agree with you in that it's probably not legal rules as written, however its just not absolutely clear to me why.

Please feel free to explain why I'm wrong, I'm just not seeing it as clearly as you are!

"the spell" is not "the spell's effect"

If the familiar delivered "the spell's effect" you could twin it fine. It doesn't, it says "the spell", meaning the whole spell. So either you cast the whole spell where you are or your familiar delivers the whole spell where it is. The metamagic doesn't this, it only changes the number of targets.

Say I twinned fire bolt. Would you allow on iteration to come from my arcane focus and one to come from my other hand? If yes, then how is this any different? The way I view it is that normally, your range for touch spells is, just that; touch range, aka 5ft. However, when you have a familiar, it expands your touch range to a similar area around the familiar. Would this arcane feat be difficult? Yes, thus the arcana check, because splitting your focus with magic, even for just an instant, is dangerous. Could it lead to some epic moments? Hell yeah! And if they fail, or even CRIT fail, well, as a DM, I personally LOVE to play around with crit fails. 

Isn't better casting Duh_Plee_Cee_Tee spell on the familiar ??

Then, while this spell still lasts, try casting any spell using all the Familiars as the casters.....

TADAaaaaaaaaaaaaaaa  !!!    is it a Hack or is it a fake ??

Quote from Dertinus >>

Isn't better casting Duh_Plee_Cee_Tee spell on the familiar ??

Then, while this spell still lasts, try casting any spell using all the Familiars as the casters.....

TADAaaaaaaaaaaaaaaa  !!!    is it a Hack or is it a fake ??

 
There is no spell called Duplicity.

There is a Trickery Domain feature called Invoke Duplicity which only affects that cleric and cannot be applied to others.

Quote from Gay_Bard_Necromancer >>

Say I twinned fire bolt. Would you allow on iteration to come from my arcane focus and one to come from my other hand? If yes, then how is this any different? The way I view it is that normally, your range for touch spells is, just that; touch range, aka 5ft. However, when you have a familiar, it expands your touch range to a similar area around the familiar. Would this arcane feat be difficult? Yes, thus the arcana check, because splitting your focus with magic, even for just an instant, is dangerous. Could it lead to some epic moments? Hell yeah! And if they fail, or even CRIT fail, well, as a DM, I personally LOVE to play around with crit fails. 

 Unless your other hand was sliced off and is currently not attached to you, it still counts as "you."  If it has been sliced off, it counts as an object and is no longer part of you.

Your familiar is a separate creature from you.  It has it's own hitpoints, makes its own saving throws and skill checks.  It is not you.  You get special abilities that let you do things like yous your familiar as the point of origin from a spell, but the wording is very clear- choose yourself or choose your familiar but you can't cast a single spell and have it fire from both.

Quote from 6thLyranGuard >>

Quote from Gay_Bard_Necromancer >>

Say I twinned fire bolt. Would you allow on iteration to come from my arcane focus and one to come from my other hand? If yes, then how is this any different? The way I view it is that normally, your range for touch spells is, just that; touch range, aka 5ft. However, when you have a familiar, it expands your touch range to a similar area around the familiar. Would this arcane feat be difficult? Yes, thus the arcana check, because splitting your focus with magic, even for just an instant, is dangerous. Could it lead to some epic moments? Hell yeah! And if they fail, or even CRIT fail, well, as a DM, I personally LOVE to play around with crit fails. 

 Unless your other hand was sliced off and is currently not attached to you, it still counts as "you."  If it has been sliced off, it counts as an object and is no longer part of you.

Your familiar is a separate creature from you.  It has it's own hitpoints, makes its own saving throws and skill checks.  It is not you.  You get special abilities that let you do things like yous your familiar as the point of origin from a spell, but the wording is very clear- choose yourself or choose your familiar but you can't cast a single spell and have it fire from both.

 The way you phrased that made it sound like I had cast scorching ray, and was asking for 2 bolts to come from me and 1 from my familiar. That is absolutely not the case. Scorching Ray is like a Trident; one weapon, 3 points of damage. Twinned spell is like taking a mold or that weapon, (say, a spear) and crafting a new one. Would I be forced to wield both weapons, just because I have the original? No. I could give one spear (spell) to my friend (familiar). Again, it would be difficult, even more so for higher level spells, but not impossible. This should have a DC high enough that trying this is only a last ditch effort. 

Would it be strong? Yes. Would it be fair? Not really, but sorcerers are the only casters that can blaitantly cast spells in public without getting caught. Would it be easy for the PC? Again, hell NO. This is a very niche thing to require, so I don't think it would be too much of an issue. Sorcerers are very much gimic characters, meaning that things like this are what they specialize in. They have an understanding of the weave far closer to that of elvish high mages.

To cast a spell, a common mage rips power from the weave, shapes it, and releases it. An elvish high mage, instead takes the weave itself, and folds it into the desired shape. Let's say they were both tasked with magically creating a Dove. A common mage would pluck a few streams of power from the weave, and shape that into the Dove. An elvish high mage would shape the weave itself into a Dove, and let it separate from the weave on its own. Much more difficult for lesser spells, like magic missile, but much less damaging to the weave for spells like wish. Also, when casting wish with elvish high magic, there would be much less chance of it backfiring, because instead of riping paper, you're folding it. However elvish high mages guard their secrets very well. If it where a class in 5e, gaining one level in it would take at least 5 years. 

Sorcerers are much closer to elvish high magic, because their existence itself folds the weave similar to elvish high magic, meaning that, though lower level spells are enherently more difficult, great feats of arcane prowess are easier. 

That was very poetic, but unfortunately absolutely none of it was based on the rules of how Twin Spell or casting through a familiar actually work.  Barring Magic Weapon, a spell is not a spear, you don't just hand it to your friend unless the spell's description specifically allows that.  Twin Spell does not work that way, casting through a familiar does not work that way.  Therefore, the argument still fails.

https://www.sageadvice.eu/2018/06/01/can-a-touch-range-spell-with-multiple-targets-be-used-on-one-target-the-caster-touches-and-one-the-familiar-touches/