I have a special shower that automatically maintains the temperature - so if someone puts the washing machine on mid shower, the pressure will reduce yes, but the shower will adjust the temperature to avoid you getting scaleded... which is nice
I only have one shower, but I'll have two next year along with a new boiler, but I'm expecting it not to work that well if two people want to wash at the same time.
Question: I know you can't get a pumped power shower if you have a combi boiler, but can you put a pump on the water supply that feeds the boiler? Obviously this would rely on a boiler being able to heat a higher flow of water, but if you had such a boiler, would that work?