My wife and I were in a partnership with my brother and his wife when we got our boat 4 years ago--first time for all of us. We bought them out last year, and it was probably a year too late. At the beginning, the only principles we stated were: 2-year commitment, and we split all expenses.
Scheduling was never our problem. What we ran into was diverging levels of interest as we all learned what it meant to own a 20-yr-old boat. I was most involved and did virtually all the work, happily for the most part, but that meant I had the best idea of what needed to be done, which wasn't always so obvious to the others. This was magnified after the 1st year when the other party sailed only twice in a season.
I definitely agree about having a buy-sell agreement in your setup. This isn't a pessimistic thing, but merely a recognition that people change, interests change, understanding changes. One person may fall in love with the boat, the other might think it's just a boat.
Keep the communication lines open, have regular "state of the union" meetings. Recognize that the feelings of ownership might be unbalanced--especially in a case like the poster who already owns the Lagoon. Will the partner feel like a partner or a visitor?