I have a deposit to return to co-tenants who were a couple when they rented and have since split up. They gave no instruction on deposit contribution or disputsement in the agreement. Am I to assume a 50/50 split and leave them to deal with any inequities?

If both signed the lease, then yes that is how I would do it.

If only one signed, I would return the deposit to only that one.

You need to do ONE check payable to them both... their splitting up is not your problem.