Multiple people, same chess player (otb)

This is entirely fictional, but what if we have twins a and b. B is higher rated than a so during a major swiss tournament (i.e) world open. Twin B plays for twin a. If they were caught, would that be a sanction of twin a's uscf, twin b's uscf or both?

Wow, that's an interesting question. I'll be watching eagerly for an answer.

Depends if twin a is aware that b…

It doesn't matter if it's twins. The same would apply if player a stepped in for player b. What does the USCF rulebook say about it? You would think this sort of scenario would be covered.

Why wouldn't player b play as them selves

Because there could be cash prizes

There are people that sometimes will impersonate a friend at a tournament across the country where nobody knows who they are or what the friend looks like 🤷‍♂️

Playing under someone else's ID would likely get you banned from playing US Chess events and possible the other player as well, if they new you did it. In the case of twins it might be hard to prove though.
Also, depending on where it was done, you could be charged with fraud, especially if money was won at the event.

I have played both identical twins in the same tournament before. Their ratings were close together.

Wouldn't you need to provide some sort of ID before winning a cash prize? I've never been in a cash prize tournament so I wouldn't know but it would seem logical that you verify the winner is who they say they are.