Came into this thread a little late, but you do know you can mount the sdcard to the computer while in sprecovery? You can then rename the theme update.zip and load the rom and rename it update.zip. Unmount sdcard and then install rom and go from there.
Okay, curious here: Are you saying to put two different files, both named update.zip, in the same directory (root) of the same drive simultaneously? I'd be a little surprised to see Windows allowing that. It's an easily confused OS
Sorry, just reading through random threads before I start messing with my phone.