Originally Posted by Joyce:
Make the contract with ONLY ONE of the parents. State very clearly in the contract that this ONE parent is the one who will physically hand you the money. This ONE parent is the one you will go to when payments are late. This ONE parent is responsible to adhere to the terms of the contract. 
This is how I do it, too.
It is the only way to keep their life decisions from effecting yours.
Now, I do discuss the childs milestones/behavior with both and also split the art projects between the two equally.