This isn't a marriage, and if you want to use the analog, then it sounds like the "child" is still-born and really wasn't the original posters to begin with. The time to leave is now. I doesn't get cheaper, easier, or less legally entangling, later on.

Hire slow and fire fast applies to co-founders as well as employees. More, even. An employee can drain a lot of value from a company, but a co-founder can destroy it single-handedly.