Actually, how I work in a collab is kinda how kokos explained.
Basically you need to agree about the final result, and what's the goal of your map before starting anything, the really good stuff about a collab, it's that the work from the other inspire you a lot, when someone has an issue with his block-out/detail, the other can work on it and make awesome things, then it inspire you, etc.
Be aware that two guys who know their stuffs don't have the same opinions about how to do something, so be sure that your way of designing, your tastes, and your skill barely match.
You'll be prompted to do a lot of concessions to please the other, if you don't have the same opinion about how this brush need to looks, then talk about it, and be honest, if you think that the other got a better argument, then go with his point of view.
Collabs are really cool, don't compare how much you made and how much he made, if his works inspire you, he could have made 35% of the thing, it doesn't really matter if the final result is even better than what you expected.