paldepind: Thanks hyperagathon, I've added it to the list.
I've stumbled into a nasty issue - when trying to play a player vs player game, the joypad ends up controlling both players. To make it even worse, this didn't happen in the Japanese version of the game (BTW, why the hell is this version ~100 MB smaller?). I've googled and discovered that Wine pretends it's two devices - so one solution would be deleting /dev/js* and/or /dev/input/js* (for me it was only the latter). However, this means you have to replug it if you want to play a Linux native game with it. After a few hours of trying different things out (the wine didn't help :p), I've come up with the following: assign P1 the P1 joypad buttons, but leave it all on "Not used" for P2 _except_ for one of the analog keys - for whatever reasons, assigning P2 Button8 to it "fixed" the problem. What if you actually have analog sticks? Stick to the first solution, I guess.
I hope this saves someone wanting to beat up their younger sibling in GGX2 some time.