Start how you mean to finish. The syndicate needs to establish on commercial terms and that way now one feels they have been taken advantage of. We are all friends until it comes to money so establish a fair market price for the syndicate shares. Alright, the aircraft is worth $200k, but is the a price it would really achieve if put on the market. How much does Owner 2 think it is worth to him? It could be the half syndicate share is established somewhere between $100k and $200k, say $150k. This might realistically be all the plane could be sold for less a discount for convenience of the syndicate. So Owner 2 pays $150k for his share knowing he has bought in at some discount. Owner 1 has established a more suitable value and will be less likely to feel he has been taken advantage of. The ongoing relationship between Owner 1 and Owner 2 has greater chance of remaining harmonious as everyone feels they have committed in a business-like manner.
Just my 2c worth.