I could see some potential problems with a MH partnership.
I'm in a partnership with my brother in an airplane. Just like noserider said, the plane is easy, we pay for our own fuel, it stays in my hangar at my house, we just split the maintenance costs right down the middle.
He & I also own some equipment together. This one is a little different. We split the major repairs, but, if one of us is using it and there's a minor problem, ie hydraulic hose, flat tire, (2 or 3 hundred bucks) whoever is using it at the time just pays for it & goes on. A major issue or routine maintenance, just split it.
Oh....we don't have anything written down either.
Our wives aren't involved.......... so it works just fine.