If you add anything else you are going to need to change the supply lines from 1/2" to 3/4" from the water heater to the shower. It also involves using a high volume pressure balancing valve and volume control valves for each feature. You will also need a water heater with a large enough capacity to keep up with the demand.

Thanks Much for your reply! Water heater replacement is in the works, but is on the OTHER side of the house, downstairs, so new supply lines are out.