My idea was more that one cofounder probably had an original idea, but then got another person to work with them on it. In the best case both people are working on the project in large part because they want to work with the other person. The project the two work on probably evolves enough between the two people that at the point they get to talking to an Angel, they can both claim full half credit and feel like equals in the endeavor. People who can do this are much more likely to be able to grow a business quickly like Y-Combinator tries to do.