In my experience, the initial idea for a new product usually comes from a single entrepreneur, but the implementation plan for a new business requires a team, or at least a co-founder. The reason is that any one person rarely has the bandwidth, interest, or skills to manage all the tasks required to build a business. Thus I find that two heads are usually better than one in a startup.
Way back in the early eighties, I had the privilege of working with Bill Gates and Steve Ballmer, when Microsoft was still a startup. I realize now that these two were near-perfect co-founders, with Bill having the technical passion, and Steve bringing the business experience from his prior stint at Procter and Gamble. Their skills and interests were complementary, and their trust in each other was palpable.
Image: http://blog.startupprofessionals.com
