I believe this is a feature request, unless I've missed something in the documentation.

Right now I have to run exiftool once for every file I want to write tags for since the GPS tags I'm writing are different for each file. Is is possible to do this with one execution of exiftool instead? The overhead associated with one call is rather substantial (at least on Windows, but I think the same was true when I used it on Linux).

right now:
exiftool -GPSAltitude=120 one.jpg
exiftool -GPSAltitude=173 two.jpg

boy it sure would be nice if this did the same thing:
exiftool -GPSAltitude=120 one.jpg -GPSAltitude=120 two.jpg

Thanks,
Tim Kostka