Why does the flow of water from a fauset decrease when someone opens another valve in the same building?

Answer 1

This happens because the water heater is in the roof or in the basement. When you turn a tap it takes water up or down from the heater down a single pipe. When you turn a second a tap the water being taken down that single pipe is slipt down two pipes and so less water gets taken to a single tap as its being shared.