Startup founders get tired now-and-again. Knowing a co-founder is working while you're resting is a source of invaluable relief against the pressure to get things done.
Long-term, this kind of emotional stability prevents burn-out. That's important because startups don't die, founders quit.
A very valid point. If you are a solo founder, nothing happens unless you make it happen and as you experience the downs it becomes very easy to lose momentum. Co-founders can provide a good counter balance and keep progress steadier.
It has been my experience that, while working in a group, it's actually harder to get things done. You have to reach a consensus first before you can act. The progress may be steadier, but in the long term slower.
Maybe I'm suffering from a bad college trauma. During college, we were forced to work in project groups. This was always a disaster, I remember calling group members at 2AM to email their contributions to the project, only to find it full of mistakes and largely copied/pasted from the internet. I'd spend the rest of the night editing the piece 'cause I sure as hell wasn't going to flunk on their behalf. When I would do a project on my own (my Thesis, for example) I'd get substantially higher grades with less hassle in less time.
I don't think college projects correlate well, for the main reason that you often can't pick your teammates like you pick your cofounder. The motivation is a grade, not money, and there are other grades (distractions) you must concern yourself with. With a good cofounder (or even coworker) there will be division of labor and trust to speed things up. And hopefully you'll see how you work together before you commit.
Have you ever noticed how few successful startups were founded by just one person? Even companies you think of as having one founder, like Oracle, usually turn out to have more. It seems unlikely this is a coincidence.
What's wrong with having one founder? To start with, it's a vote of no confidence. It probably means the founder couldn't talk any of his friends into starting the company with him. That's pretty alarming, because his friends are the ones who know him best.
But even if the founder's friends were all wrong and the company is a good bet, he's still at a disadvantage. Starting a startup is too hard for one person. Even if you could do all the work yourself, you need colleagues to brainstorm with, to talk you out of stupid decisions, and to cheer you up when things go wrong.
The last one might be the most important. The low points in a startup are so low that few could bear them alone. When you have multiple founders, esprit de corps binds them together in a way that seems to violate conservation laws. Each thinks "I can't let my friends down." This is one of the most powerful forces in human nature, and it's missing when there's just one founder."
Long-term, this kind of emotional stability prevents burn-out. That's important because startups don't die, founders quit.