In the first Trine, as well as the normal mode for upcoming Trine 2, the co-op is class-restricted (by which I mean players can only have one of each class in a game at a time). It makes sense, as the whole magic of Trine is to use the different abilities of the characters to navigate everyone through the various levels. Just this past week, though, developer Frozenbyte announced that Trine 2 will also have a special "unlimited mode." This mode, separate from the main campaign, will allow players to play as any combination of classes they want. Always fighting over who gets to play the rogue? No problem, just bring three in unlimited mode!
They've also provided a short video showing off the new mode:
Trine 2 will be out on XBLA, PSN, and PC with 3-player co-op later this year.
Source: Siliconera.com