It's like the classic "level or life" way for more than one person to enjoy any single player game, except the game has a pointless screen which tells you whose turn it is and doesn't offer any other multiplayer feature apart from that. May as well not be there at all, IMO.
E: In fact that probably doesn't really count as being "multiplayer", so to speak. When I did PS1 Abe's Exodduss with Alector and PS1 Abe's Oddysee with Middlesboogie (old OWF veterans who no-one remembers) level or life style for fun, we didn't bother with 2 player mode.
It'd be nice to see an add-on or a whole game dedicated to a co-op campaign, with specially designed levels for it, like Portal 2's multiplayer. Although there's a lot I'd rather JAW worked on first. Like AE HD. Or probably Sligstorm.
E2: What was fun was two people playing a copy of the game at the same time in the same room (had to be on the same platform, otherwise it wasn't fair) and racing head to head to see who could beat the game first.