That is why he is a salesman instead of a plumber....

First off,both showers are initially controlled by the same tempurature mixer and they install a diverter on the output side of the mixer so you can select shower, rain shower or both.

Each shower head is rated for 2.5gpm so the combined load is 5gpm.

The water heater produces 120degF hot water while a shower is typically 105degF. The mixer has both a hot & cold water line and typically we use 87% hot & 13% cold water to produce the desired 105degF shower, but for the sake of discussion let us assume you want a super hot shower at 120degF so you will only use hot water, which means all the water is conveyed in one pipe.

The resultant pressure at any fixture can be computed by begining with the static head pressure at the house main water shutoff, then deduct VSHL (vertical static head loss) & FHL (friction head loss) which is the resistance to flow caused by friction with the pipe walls.

We don't know the layout of your house so let us assume a worst case scenario where the water enters the house near the floor in a basement and the desired shower is on the second floor. In the basement we have a vertical rise of 8ft to the first floor, another 8' to the second floor and a third 8' to your ceiling mounted rainshower head for a total vertical rise of 24ft.

VSH is a constant of 0.434psi/ft vertical so the VSH loss to the rainshower is 0.434psi/ft x 24'=10.4psi

To derive a finite answer I would need to know the size and length of all lines from the main water shutoff to your shower, but for the sake of illustration let us assume a worst case scenario where you have 120' of 1/2" copper. To derive a finite answer I would then need to know the type of all fittings on that line to determine the fitting insertion losses, but in residential work code allows us to use a rule of thumb shortcut, thus to derive TDL (total developed length) we can multiply the physical length by 1.20, thus a 120' line would have a TDL rating of 144'.

1/2" copper pipe with a flow rate of 5gpm has a FHL of 0.161psi/ft which would be a 23psi loss.

We then combine all the losses. VSHL 10.4 + FHL 23psi equals a total pressure loss of 33.4psi.

Code minimum for static head pressure at the main is 45psi but in the real world municipal suppliers generally supply between 50 & 60psi.

Keep in mind that all of the losses above are computed on a worst case scenario and in your house those losses would be much less, but even at the worst case with 45psi at the main, when we deduct the total loss we still have 45psi - 33.4psi equals 11.6psi at your shower and the ASTM (American Society of Testing and Materials) specification says that all showers must function at full flow with a minimum of 8psi.