Why does the flow of water from a tap decrease when some one opens another tap in the same building?

According to property of water, in a certain floor of a building rise of ammount of water is fixed, i.e., in a certain pressure, ammount of water in a certain floor,which comes from underground by pumping machine, is constant. So when you open one tape you get more water but when you open two taps total ammount of water on that floor is distributed in two taps. That is why speed of water on the 1st tap decreases.