You own 50% of the player and pay half his wages, that player then can either play for team A or B until 1 year (or two depending on the contract you make with other team)
at the end of the year both teams bid for the other 50% of the player, with the higher fee winning.
This gives you an opportunity to essentially take someone on a trial/loan and still get him or withdraw your bid for him if he isn't good enough.