I know it sounds silly but as a startup grows, later in the process investors will want you to have a 'co-founder' who could basically take over if the original founder couldn't for any reason. It's de-risking. Probably needs a different title than 'co-founder'.
A one-hour collaborative problem-solving interview is a great idea.
But, the challenge I have is that I want some kind of measure of aptitude that isn't subjective. I've seen that founders tend to overestimate their ability to basically 'vibe' someone based on interview and the point of this question is to try to hedge that.