I'm afraid I can't help too much either except to say that we considered entering into a shared ownership agreement and decided against it. It meant buying an older, less expensive boat but we are very glad that we made that decision.
For us much of the joy and satisfaction of owning the boat has come from the "at the dock" stuff rather than the actual cruising time. Cleaning, repairing, upgrading, experimenting, just "hanging out" on the boat - all of those activities would be tough to sort out in a co-ownership.
I'm sure co-ownerships can be made to work. As Rock says definitly have a lawyer help with the details.