This is not a stupid question. What you want to do here is
very advanced. So advanced in fact that exiftool does not
have this ability. Currently, the -if statement always
applies to tags in the source image, which is the same image that
will be edited when writing tags.
The only solution I see to this is to write a batch file to do the
work for you. In Unix, this would be very simple to do, but
if you are running Windows I can't be much help.