I agree with tyrrp's second suggestion: You ought to have two similar looking devices, have the user select the right one as he starts to watch TV in each room. Even if most of the buttons are identical (control a shared SAT, CBL or DVR device, etc.) the different vol commands are enough to justify a different device.

BUT, though I don't reccomend doing it, I do know how to do what you asked, I'll probably even help if you like. In most cases you can merge the pronto hex of two unrelated repeating signals so they will repeat together. Post the pronto hex of both vol+ signals and both vol- signals and I'll tell you what to try for merging them.