Are you wanting both shower heads to be on at the same time? On my old shower, I had connected a hand shower to a wall union in the place of the tub spout and the "diverter" was really an A/B valve that selected between fixed shower and hand shower. When I redid my shower, I got rid of the diverter and just connected the single shower head to the tub filler port.
On many of the rough-in valve bodies, the shower port is smaller than the tub filler port. Depending on the GPM on the shower heads, there is the possibility that the tub port may steal some of the flow from the shower port.