That's a good point. Startups are not about exits, but big exits.

I wonder if the $$ stack up, and where the sweet spot is, given 2 founders need twice the money from an exit.

Usually a tech and a non-tech co-founder partnership works really well because you cannot scale up if the person building the technology/product is also building the marketing and sales machinery.

Single co-founders only work in domains where either of those two - technology/product and sales/marketing - are much less resource consuming than the other so that one person is able to focus on both. That model breaks at scale.