Orcs Must Die! 2 is more than just the original game with co-op. It's a refined experience, improved in just about every conceivable way. A greater variety of enemies with more unique sub-sets have been added, along with new traps to deploy, and a completely revamped skull system (enemies sometimes drop them now).
The addition of co-op means you often deal with more enemies and feel more pressure during each wave. Utilizing the two protagonists with their differing abilities and trap-sets in a strategic way is paramount to victory.
Yes, each character has access to a specific set of traps exclusive to them. It helps each stand out beyond their other major differentiation: the War Mage -- featured in the first game -- has lots of health and little mana, while the Sorceress is the complete opposite.
The Sorceress can spend time summoning skeletons and other allies to occupy the horde while the War Mage unloads shot after shot, making for an interesting co-op dynamic. With an alternate fire function, the War Mage can turn his gun into a grenade launcher, which is entirely necessary for dispersing crowds.
Outside of the variety and volume of enemies we had to deal with in each wave, and the inclusion of a co-op partner, Orcs Must Die! 2 played a lot like its predecessor. For me, that's far from being a bad thing.