Overhead shower + handheld without pressure loss

[KillerCharlie]

I'm replacing my bathtub with a large shower and want to put in a regular showerhead plus a rail-mounted handheld against the same wall. The catch is that I want to be able to run both without a huge flow rate loss.

Systems like this definitely accomplish that, though they require a separate diverter and the cost really adds up... nearly $600, ouch.

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There are tons of showerheads + handhelds on Amazon with external diverters on the showerhead that are much cheaper and easier to install (less cutouts in the wall). However, from my experience the diverter usually reduces the flow of the entire system, so you can't have both running with full flow rate. E.g. you get 2.5 gpm and have to choose how to divide it up. The diverter valves are very difficult to modify to increase flowrate (unlike the showerheads themselves). Here's one example: https://www.amazon.com/Waterpik-XET...al+shower+high+pressure&qid=1616289569&sr=8-6

If I want something that can do both showerheads at nearly the full flow rate, is my only option to go with something like the first system I listed?

 

[Micrornd]

If you look carefully you will see that the Delta uses 2 separate supplies, while the Waterpik uses only one. This allows the Delta to maintain a higher volume of flow in each head, while Waterpik is limited to what a single supply can provide. Granted both are ultimately coming from the same supply line behind the wall, but it does make a noticeable difference.
For a shower with multiple heads you also want to make sure that the supply line in the wall is adequate, 1/2" in marginal, while I've seem high end units in custom homes with a 1-1/4" supply running 10 heads!
You can of course increase volume supplied by each head by removing or drilling out the flow restrictor in the head, just go in small increments as a little goes a long way. (This is not for you Greenman, as they throw you in jail for doing that in Cali )
Line pressure also adds into the perceived "feel" of the shower and should not be overlooked when trying to work with an existing installation.