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.

https://www.build.com/delta-dss-lahara-1701/s973688?uid=2415146

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-633-643-Pressure-PowerPulse-Combination/dp/B07GBN8NCK/ref=sr_1_6?dchild=1&keywords=dual+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?