If you want a shared ownership, one player would have to be owner, whilst the other would be a 'lot operator', they would have all the permissions of a lot owner. To create a lot operator, type /allow !<username> and their name will appear as a darker purple in the lot information shown via /thislot. This is the best we can offer you in the way of two players owning one lot, however please don't exceed your lot limit by doing this.