I wonder how often fallen-out relationships between cofounders get reconciled. It seems like the force driving change needs to be pretty compelling... something much larger than the two individuals.
I think they both love reddit. I know as a user I love reddit, and despite the myriad problems with it, if I was in a position to give it some TLC and make it a place I would want to be, I would put aside interpersonal differences to make that happen.
Professional fall-out tended to be due to professional reasons. If the root causes could be fixed or overridden with a greater cause, why could they not work together again?