I'm not 100% on the full idea of it, but I know that players can be owned 50/50 by two clubs, playing for both at different times (I think) so when a team buys said player, they have to buy the half from each club.
If a club wants to buy a 50/50 co-owned player, they have to negotiate transfer fees for each club that owns 50% stake, and then negotiate an acceptable contract with the player/agent.
It's usually a huge pain to try and buy a co-owned player, I've tried before and often 1 of the clubs jacks up the price by a lot (because they were previously interested in retaining the player, hence why they bought a 50% stake).