Don't confuse pressure and volume...a 1/2" shower valve can usually flow about 5-6gpm. So, what is the rating of the combined two showerheads? As you try to exceed the maximum volume, you start to lose pressure. Think a hose sitting without a nozzle, then try to decrease the flow by putting your finger over it. The supply pressure doesn't go up. In order to have the showerhead feel good, you need to be able to supply more water than it can dispense, or it's like you took your finger off the output of the hose.