How do I Format Simultaneous Dialogue?

A Reader's Script Formatting Question:

How does a writer denote in a spec screenplay the fact that a character has a double identity and is known to individual characters under two separate identities? Example: a character is known as 'BILL' to one set of characters, but 'JIM' to another -- do you type both BILL/JIM each time he speaks dialogue in the screenplay? Bearing in mind that the crux of the story is that he appears as one person to one set of characters, and another person to another set of characters! -- Complicated, I know but I would appreciate any advice you are able to give as I am having serious headaches dealing with this!

Dave Trottier, author of 'The Screenwriter's Bible' and the upcoming 'Dr. Format Answers Your Questions' responds:

You ask a good question, since it will be important to not confuse the reader. Clarity is the overriding principle in cases like this one. That is why you should normally use the same name as your character cue throughout the screenplay. Thus, I believe the best solution is the one you suggest. Refer to the character as BILL/JIM in the dialogue character cue whenever he speaks, as follows:
BILL/JIM
What did you just call me?

Now if this character's true identity is BILL and that's established early, then consider referring to him as BILL, even though some characters might call him something else. That's what happens in NORTH BY NORTHWEST. We know that Cary Grant is Roger Thornhill, even though most people call him by another name throughout most of the screenplay.