Now I have only 1 USB gamepads... I broke one when playing virtua fighter evolution.

My hint is:
A) Make your game turn based
or B) Use the combination of keys for several actions:
for example: A turn left, S turn right, A&S move straight. There are multiplayer games in the web which work like that. Its fantastic, because they knew that problem but wanted to make a mega multiplayer game playable on one PC. That means, the used for player one keyboard, for player 2 keyboard for player 3 mouse and for player 4 joypad. The controls work like that: key 1 = move left, key 2 = move right, key 1 & 2 jump

That means the player with mouse can walk left when pressing mouse 1 and walk right when pressing mouse 2. When he presses mouse third (some horrible mouses have no third button - so they made the mouse 1 & 2 to jump) he jumps.