Tim -
Think about the issues here - two copies of the same image running, recording changes to the same changes file. You open the changes file for write from the first image to get to a point where it records a change. You try to open the changes file again for the second image. Does your OS allow opening a file multiple times to write? Does it even allow opening to read once it's already open for write?
Actually it allows all of them. I made the decision for having an exclusive write lock on files opened with read-write since Squeak doesn't give us much control about the intended behavior and in the past I was bitten many a time by having different programs (or even the same program!) write to a file concurrently. This can actually be a problem if you know what you're doing but unless we have better control from Squeak directly I don't see any better solution.
Cheers, - Andreas