In the beginning, there were two. No, I don’t mean Adam and Eve. I’m talking about so many startups that become successful — they are hatched not by a single visionary but by a pair of them. Though I’m hardly the first to make this observation, choosing a co-founder is akin to choosing a spouse. You will live, breathe, fight and rejoice together, often in close quarters and often for a long time. You want to choose wisely. You want to find a keeper.