is through word choice. As we "age" our vocabulary tends to change. Also, even if we know words - our choice of which to use is colored by our experiences. You could have one "incarnation" speak with a more mature voice. Also, if it's in first person - just pick one incarnation. Then it's:

"well, I guess you might have a point." she said.

"Of course I do." I replied.

That kind of thing works - when there's two talking. You just have to make it clear which of the incarnations have POV.

Another way to handle it, is to have the two versions discuss what to call each other. For example... Were I to meet my teen self, I'd expect to be addressed as Ms. MacGregor and I'd call her Ann. (Okay, this isn't fully true. I didn't go by Ann or Annette as a teen. *sighs* And, I've not transitioned as yet, so the younger me would probably use Mr. X.... and I'd use "his" boy name. *sighs*). Just depends on the age difference, and whether the younger Terri was brought up to be "formal" to adults.

Obviously, the two of them - when alone - have no trouble. It's only others interacting with them, and the readers, that get confused. So, you could have a 3rd person get exasperated by both of them responding when the name's called and let this 3rd person decide... "Enough of this. From now on, YOU are Terri and YOU are Ter." or some such. The one getting the "name" would be the one the 3rd person knows better.

My 2 cents... Good Luck.
Anne