One of the biggest reasons solo founder projects fail more than multiple founder companies is that a multi-founder company has already passed some tests of market, product, and team.
If an idea is incredibly stupid, but it's your idea, you can easily become deluded into thinking it's actually a great idea. Being able to convince another person that the idea is good is definitely a sign that the idea is good.
It's also a good check on the quality of the team -- if someone is obviously such a screw up that you'd never want to work with him, you wouldn't work with him. Even fairly defective people can notice flaws in others. Therefore, a group of 2+ people probably means the members are at least tolerable.
If an idea is incredibly stupid, but it's your idea, you can easily become deluded into thinking it's actually a great idea. Being able to convince another person that the idea is good is definitely a sign that the idea is good.
It's also a good check on the quality of the team -- if someone is obviously such a screw up that you'd never want to work with him, you wouldn't work with him. Even fairly defective people can notice flaws in others. Therefore, a group of 2+ people probably means the members are at least tolerable.