Two shower heads - Coop allows?

so stop whining and move to the burbs already

obvi your call, incredible waste of water.

If you want to tear the wall up to install all that, then it counts as an alteration and you do need to go through the rigamarole.

Or you can replace your existing showerhead with a hand shower and rain shower and be done with it. Not as pretty, but that way you can see whether it's worth doing without a lot of expense.

If it's a stall shower rather than a tub, check whether the drain will be able to handle the increased flow. Just take off the showerhead and let the full flow come out of the pipe.