Ask HN: Have you ever cofounded two startup's at once? How'd it go?

I have a unique opportunity in that I am involved in the founding of two separate projects. One of them I am cofounder with another partner, we're preparing to go into beta soon (social space) and have worked on for 1.5yr+; it's my baby. The other one has sprouted up out of the consulting work both myself and partner have been involved with to help fund us working on first startup. The CEO of a successful & rapidly expanding company that is backing the consulting project wants us to cofound a much bigger picture project in that space with him. They both have a ton of potential and are exciting in their own ways. We're also already testing the first phase of software for the 2nd project internally with said company and seeing success. Feedback from initial users of social app is also exciting...

My question is this, I'm 33, no wife/kids, no mortgage, and highly driven; but would it be sane to be involved with two projects? I am the ceo of social project and as it's my first startup and first time leading such a project, I want to have the best chance for success. Likewise I would need to figure out my role for second project but so far I have acted like a mix between leader & developer in coordinating other contractors, developing on backend/client, and interfacing with company backing it. I'm having a hard time figuring out what approach to take if I choose to be involved in 2nd project and also have FOMO of ditching it entirely. I'm also interested in approach from angle of myself and partner from first startup as a team etc and if it makes sense to spread the risk around some in two projects?

Anyone ever have similar situation? Advice? Thanks!