Owner two sells the share for what he can get for it, simple. I've made a small profit on syndicate shares I've sold in the past. It's none of one's business what the share is sold for. I'm surprised there's any debate?
And I've taken a small loss selling syndicate shares in the past.... How is that working out for you?
The shares of any syndicate are only worth what somebody is willing to pay for them. If owner 2 buys 50% of the aircraft for $50,000 and then later decides to sell it and gets $100,000 for it then owner 2 has made a tidy profit. Much like my mate the ranga here