Back in the 1970s when I was single, my dentist (also single) and I bought a Piper Commanche together. We laid out rules regarding maintanence, expenses, insurance, who had priority on using it, etc. and everything worked out very well.
THEN....he got married and his wife didn't like flying and demanded he sell his half. That is when it turned bad. He wanted to sell to the first person that said yes. I wanted to have a say so on who he sold to since it would now be MY new partner.
After a lot of angry back and forth, we both ended up selling the plane to one party. I bought a Cherokee by myself and said "no more" to partnerships. He and I ended up no longer friends and one year later he got a divorce.