From Sid's link:
Usually, keyboards will report as many unambiguous keys as they can. However, the standard reporting protocol limits the number of simultaneous keys to six QWERTY keys plus the modifiers (such as shift, control, and alt).
, does this mean that with most keyboards you can actually press 9 keys (6 + 3 modifiers) at the same time, even if it would be a bit weird stance?
Anyway, if I were you, I would check older games to see how they solved the problem of having multiple persons playing on the same keyboard (anyone remember jump & bump with the rabbits? you can play it with 4 persons together on the same keyboard iirc).
Than there are the SNES and Neogeo simulators with e.g. beat-them-ups where 2 players can play on the same keyboard.
Perhaps another possibility is attaching 2 keyboards to the computer (keyboards can be very cheap), maybe that would work even though it would be a bit akward
.
