Plan for the divorce during the honeymoon.

Then put the agreement in the safe. Two outcomes:

  * everything goes swimmingly: you've wasted a bit of time and paper
  * everything does not go well: hard decisions are already made

Before I co-founded a startup, I read the Nolo books on partnerships, especially the parts about dissolving them fairly. The details differ slightly if you have an LLC, C corp etc (as long as it is private) but the overarching themes are the same and so so important to consider.