Senior dev chiming in here (44yo), mainly replying to some other comments here, that claim once your team is > 1 person, your overall productivity does not increase with a person every time.
I would say a good 2 person team is more productive than 2x 1 developer for the following reasons:
1. It's easier for 2 people to keep each other motivated than 1 person alone.
2. When you get stuck on something, or have a difficult problem, you sometimes lose a lot of time as a solo developer. Having a second expert to consult might save you tons of time.
3. Individual developers have their own preferences and expertise. I mainly divide developers in 2 groups: bottom-up and top-down developers. Bottom-up are interested in technology, math and everything close to the hardware. They don't like squishy stuff like users. Top-down developers love to crunch out features, and technology is just a means to an end. Having a team of one top-down and one bottom-up developer is ideal.
If you are still not convinced, let me ask you the following: Which is most productive? John Carmack for 1 year, John Romero for 1 year, or Carmack and Romero for 6 months?
If you talk full-stack, is your bottom-up developer really good at the user interface, and will the top-down develop get bored with the backend stuff?