These are excerpts from a conversation I was having about the inability to form a slavery contract as a result of self ownership and I was just looking for thoughts.

There is also significant conflict towards the notion one could "sell" themselves as it would be a deceitful contract at best. Nobody can own you because you exist as yourself. This is generally what libertarians look to as self-ownership.

So joint bank accounts, houses bought by married couples, cars with two names on the title aren't owned by anybody because two people can't own the same property?

It can be but these all face the same problem really. If person A decided the car belongs solely to them, they can take off in it. Again this will likely lead to the end of the contract. The difference here is person B actually doesn't have that option. I can't take off in your body...yet. Transhumanism could offer brain jacking to allow this and that is an interesting concept to approach. One which I actually haven't considered so you can try to approach that but my answer at this point would be IDK.