Momentum is vital in getting your start up to revenue. Other business ventures I have tried and failed with have been with more than two people, or alone. Consider the power of two. Nature made it this way. Nature is smart. 
Put simply, when you have too many contributors early on, there isn’t enough contribution. It’s easy to get lost in ‘who is doing what’. It’s less accountable. It can get confusing. You turn up to the next meeting and Joey didn’t contact Timmy because he thought Andy was doing it. You get the picture.
By yourself, it is hard to maintain momentum. You need a special level of discipline, most of us don’t have. There is no reliance on one another. No bouncing ideas. No pulling each other out of a quagmire. No bullying.
I’m not saying it can’t work with one or many, it’s just harder. Starting up is hard enough on its own. With two there are no committees. You will also find out quickly if you are both as committed as each other. There is no where to hide. Actions are clear because they are decided with a person to person conversation. Markets are conversations.
Forget there is no majority. If you can’t come to a decision without a majority vote you should go and get a job with Joe Punchclock.
“It’s easy to get lost in ‘who is doing what’”
Hmmmm … Only if the project manager isn’t proficient in his/her role. The key to good project management, or any other management, is in the preparation.
Laying down clear roles and responsibilities at the outset no matter how many people are involved in your project will go a long way to avoiding the blame storming game come deadline time. The more specific and UNambiguous you are in setting the project team members’ accountabilities the better.
But I do agree that the more participants the harder the PM’s job becomes … but it’s all manageable.