Hammerfight appears to support multiplayer, as it has an option for 'multy-mouse' input. How can I actually get into a multiplayer game with a friend, and does multiplayer support mouse+gamepad input?
Getting into a multiplayer game is actually rather easy, and there isn't much difference between a controller and a mouse (though I haven't actually tried plugging in two controllers at the same time, admittedly. Mouse + controller works like a charm).
To "enter" multiplayer mode, just click on each connected mouse or press a button on the connected controller. You should then have two (or more) cursors visible (Red and green for two players):
Next, pick a game mode (either arena, grim or hammerball. The story mode does not support co-op) and have each player pick a profile (in the screenshot, player 1 has already picked a profile):
Then, multiplayer madness: