2 work fine in software that supports them. They show up as mice. Software that uses they system mouse (the pointer) doesn't support 2 guns because in windows, all mice control the same pointer.
At this point in time:
MAME supports 2 player (enable multi-mouse)
Model 2 emulator only supports one player natively, but can do 2 players with Howard Casto's Troubleshooter 2 program (this also adds 2 player to some PC games)
Demul emulator does not support 2 player.
Most online flash games are only going to support one player.