well, as said you have to make sure, that the formulars have different names, that was the problem here (at least iirc, hey it's one year ago...)
i'm getting that error with my board now, and i'm using the Catalyst style. everything worked just fine until I added a few tweaks, upgraded the board from 3.0.1 to 3.0.5 (over a span of few weeks) and added a few more hacks.
i can't figure it out... which one of the changed templates is messing it up.