Investors in general prefer multiple founders, and they want the founders to have roughly equal equity too. In part this is because they want the ability to push a founder out if he/she isn't working out.
I also hear that it's protection against a founder quitting. Though this to me seems dubious as I would guess that sole founders are much less likely to quit prematurely (for obvious reasons) than a co-founder.
From what I've seen it also smooths out interactions between founders and staff... people will prefer to talk to one founder over another for certain things.