Don't make a vow, just create an understanding: there will come a point in time where it will not be in person A's best interest to keep doing the startup, and you guys will have to deal with it amicably.

Ditto re: "communicate beliefs and disagreements". That's an extremely hard task because a lot of the time they'll reside in the emotional plane and be hard to articulate. I'd focus more on "can we resolve arguments without hating each other?"

But yeah sounds like you're on the right track. Good luck.