I've had successful exits founding companies in all three ways (solo, with one co-founder, and with two co-founders) and from my experience having two co-founders has been the best experience. One co-founder means you get a bigger slice of the pie, but also means that there's no mediator if the two of you can't agree. And take it from me, there will be many times when you don't agree. That's why I prefer two co-founders, because when two people take a side they're often able to convince the third member.