Interesting situation. Lesson is to get a market valuation done at the time of initially selling the share, and charge appropriately for half, quarter or whatever shares. Capital gain from that point is then legitimately allocated to partners, whenever they sell. In this case, if the share was advertised as a half share, and Owner 2 put in the running costs etc on that basis, he's made a good deal when he sells for current market value of the share. Owner 1 might have been a bit more prescient at the outset!