The fact that they were strangers is super surprising, especially given how YC seems to discourage that kind of "founder dating", suggesting instead you found companies only with people you trust deeply.
[0] https://medium.com/the-story-of-grip/how-two-guys-that-barel...
"The pair worked tirelessly for three months in a cramped office in Cambridge, waking up everyday at noon and working till dawn the next day. “I think we started like most of these tech companies begin. Just a couple of guys in their boxers coding in a dark room,” Houston says. “We just kept our heads down and built.”"