I have an OpenC64MegaDrivePadAdapter that also solves this problem on real hardware, but for vice emulator users it would be a nice feature, also for real hardware users, if there was native support for the second fire button.
I played in RetroArch with the VICE emulator, and mapped a second face button to Up, so I had A and B buttons as Jump and Fire. It worked really well. I assume plain VICE could do the same thing.
