I've been apart of large co-founder group (4+) which I don't recommend at all. Once our company went above 50-60 people the co-founder relationship just imploded and after 100+ employees, none of the co-founder were apart of the company and it eventually shut down by the investors who took over.
I've also had a co-founder where we tried but eventually had separation of idea and roadmap but the toll of the pressure was too much. You not only have to care deeply about the project but also the other co-founder and if one side is more into "glory" of a startup founder, then you're in a failing relationship.
As an eventual solo founder, I realized the size and ambitions of the "startup" are extremely important. Small ideas are ok but might not be exciting enough and big projects are nearly impossible to pull off. Figuring out how to recruit contractors and split the work load to achieve MVP is the trick.
Important tools and mindset you need to master:
1- Jobs to be done (JTBD)
2- Objectives and Key Results (OKR)
3- Sales pitch (not just a pitch deck or executive summary)
4- UX and technical skills (aka: jack of all trades)
5- People management (not just project/product management)
6- Open to criticism (nobody wants to tell you your baby is ugly, you have to assure them they can be honest with you and you'd want to hear the brutally honest feedback)
7- Go to market strategy (value props and business model)
8- Network effects
9- Time management
10- Business, legal, accounting, payroll and taxes