How do I create a scene break to separate instances of letter-writing between characters

I'd just hit enter twice and make a blank line. Are you asking how to program this? In fiction writing, letters have been used as dramatic devices for, gadzooks, probably centuries, and my understanding is that the formatting is just to keep them separate and clear as to who is writing what. Perhaps with italics. E. g.

To: Jane From: Edward
Marry me!

To: Edward From: Jane
You mock me!

At least you apparently understood the question he was asking, Jes. Me, I was lost for an answer, not comprehending the question.

There is no standard way that I know of.

Although you can enter an extra blank line, it could get lost if it falls at the bottom of a page.

You can try a few asterisks before and after *****

If your work is accepted for publication, tell the publisher that you want the messages visually separate from the rest of the text. The designer should be able to accomplish this.