That's a pretty interesting idea! From Steem/blockchain side it's basically all there to implement this via multiple keys or auth accounts in combination with a threshold > 1. You'd need this for at least the active and the owner permissions. Posting permissions may be possible with threshold=1, depending on the trust between the parties. What's mostly missing is the client side. There are libraries like piston and others that can pre-sign a transaction and enable to hand it over to the other party to add the missing signature(s). But realizing a ready-to-use and safe implementation thereof is still a bunch of work I guess. Additionally you have to keep track of the fund shares of each party. For example if one side decides to cashout or power-down parts of his share, then the reward sharing ratio for all coming author/curator rewards changes as well. Is this best done with an external database, or reconstructed each time from the full account history? Maybe periodic funds share snapshots via custom_jsons or alike could simplify this...