I wouldn’t say there’s a single rationale; it depends on the founders and depends on the VCs. That said, the inherent risk of conflict between founders is actually a reason to want 2 founders, because you have some of the same conflicts with 1 founder and sometimes it can’t be resolved.
One reason to want a business co-founder to pair with a technical founder is that the business side of a growing startup is a full time job, and sometimes too much for a technical founder, and/or sometimes too far outside their comfort zone. VCs also (in my experience) tend to like the CEO to have a certain mentalities about how to do business that some technical founders may lack, especially first-time technical founders. Things like willingness to pivot, views on taking responsibility, how good at pitching and sales they are in the broad sense of selling the company and the vision, and lots more stuff like that.
One reason to want a business co-founder to pair with a technical founder is that the business side of a growing startup is a full time job, and sometimes too much for a technical founder, and/or sometimes too far outside their comfort zone. VCs also (in my experience) tend to like the CEO to have a certain mentalities about how to do business that some technical founders may lack, especially first-time technical founders. Things like willingness to pivot, views on taking responsibility, how good at pitching and sales they are in the broad sense of selling the company and the vision, and lots more stuff like that.