I'm not a big fan of splitting up something and being to one designer, you come up with a concept, and you come up with a concept, and let's compare notes. It happens a lot, but I feel that that breeds mistrust, because if one person's idea gets selected, and the other person doesn't, it's a recipe for this weird, ego vibe. So I try to avoid that.
What I do is divvy up the tasks so everyone manages a certain aspect of the project themselves. I think giving someone that ownership really puts the pressure on, but also allows that person to express their point of view. I think it also makes people feel that they're a part of something, whereas if you split it up and you give the entire project to one person, an entire project to another person, it's not so much a team work anymore as it's like, who can come up with the best concept.
I think being able to strategically split up a project in a way where people are tackling different aspects of it, I think can yield really interesting solutions, and also make people feel included.