Water Pressure in Showers and Goind Cold When Tap Turned on Drainage and wastage systems and plumbing help, advice and answers

[Doitall]

We have two showers, a bath, a kitchen sink and toilet basin all run from a combi boiler. When someone is having a shower and someone else turns on a tap elsewhere, the shower goes cold. I am sure this is a common problem but can anything be done, perhaps to maintain equal hot and cold pressures in the shower, even though the rate of flow drops? Can some kind of constant pressure valve be fitted in the supply pipe(s) or is this a stupid question? Many thanks.