diverter
If the "shower" port goes to the rain head, and the "tub" port goes to the shower head, you MAY NOT have a problem since it will take considerable back pressure from the shower head to force water up to the overhead shower head. But that is not a given and if there is sufficient back pressure both heads will operate at the same time, although the rain head will just "dribble" water, except when the rain head is actually selected.