I'm not sure how you'd approach it. Perhaps make it autosize depending on the content of the panes or maybe add a handle that the user can grab to resize the divider manually?
Anyway, like I said, it's not a big deal because you can use the scrollbar if you really need to see the end of the filname/path, but I thought I'd post about it anyway in the hopes of helping out, if even just a little.
Thanks for all your work on FileBot, it has been an invaluable tool in organizing my media collection. Simply put, it is the best tool of its kind available!