Enter into a formal agreement now. Ideally this should have been done at the beginning. Before you start collaborating. Even if you're friends, or very good acquaintances, make a formal agreement stating the terms of the collaboration. Who owns the IP, how much share each of the members have, conditions for buying out the other member, the terms of the collaboration etc.
Beyond that, the collaboration should be based on trust. Why collaborate with somebody you don't trust? You're just going to spend time second guessing their decisions, instead of focusing on the product. After entering into a formal agreement I wouldn't spend too much time on this. If something goes awry, you have the legal document that both members have to abide to. I definitely suggest approaching a layer with this. They're in a position to understand better what all the gotchas are with your specific collaboration.