Showers Pressure When Running Dual Shower Heads

[hope2505]

Hi all,

I’m a DIY’er and certainly not a plumber but I am a ‘give it a go’ kind of person with the fall back of getting a pro to fix any of my problems

I have a nine year old house that has an unvented / mains pressure system.
I am re-designing my main bathroom, which currently has a bath, shower, sink and toilet. If I run the shower (which is the furthest from the tank, if that is important?) and run a tap on the sink or bath (hot or cold) then the pressure drops in the shower.

The redesign is to install a level access shower in the same location for my disabled son. The level access part I am good with and confident with the mechanism of waterproofing. The problem I have is that I want to install a rainfall from the ceiling with a completely separate handheld and I would like to run them together without any effect on the pressure. Any ideas?

Thanks in advance.