Ask YC: Difference between two founders and single founder with full time employee?

First of all, thanks to Sam for speaking at Washington University tonight. I really enjoyed the talk and like YC's drive to "increase the innovation output in the world."

The question I was going to ask during the talk was about co-founders vs. a single founder with a full time employee. I understand that YC prefers founding teams over a single founder but I'm curious to hear more of why. I am seeing first hand the amount of work a startup is (I know I had a productive day when I end it with more things to do than when I started it). But are there other reasons (company dynamic maybe?) why the same number of people would be more likely to succeed in a horizontal company structure (co-founders) vs. a vertical company structure (one founder with employees)?