What does "combine" mean in this case? Is it merging, appending, or something else?

How do they arrive? Does the remote side push them at you, or are you pulling them in? Before you do anything else, you need to know when the files have finished transferring.

I'm curious how you want to decide if one file is not there, too: it won't be there until the other one finishes transferring I'm guessing, so for a short time it looks like it might not exist. Why might a file be missing? Failed transfer? Database crashed and didn't create it? Something else?

Given more information, I'm sure somebody can come up with a way that takes those things into account.

Good luck. :)