Startups are just so much work. You need some help.

For example, if you have two founders, then one can go out and do fundraising for a month while the other keeps the company running. If you had only one founder, you'd be dead in the water for that month. (Even if you have employees -- things are going to decay if all of the founders are distracted.)

Also, I've never known YC to play founders against each other.