JWiser:...So how do you go about working on the same file?
You don't.
You work on separate copies, then one or the other of you merges the other person's code into their own workbook.
Best if one of you can work on udfs or procedures for importing data from other sources or interacting with outside systems while the other works on macros that manipulate workbook objects. That way your separate code should have the fewest number of inconsistencies to reconcile.