because of the pressure of the tap

The second tap lowers the pressure.

itive of pressure=

The water in a building has a certain pressure, depending on how high the storage tank is. One tap opening has the full pressure acting on one small area, the size of the tap. If a second tap is opening the pressure acts on a wider area, so it is not so strong.

The pressure of water in the pipes is called the 'head of pressure' and it is the difference in level between the water surface height (above sea level) in the nearest reservoir to the height (above sea level) of your tap. This 'head' will be higher for a downstairs tap than an upstairs tap and therefore the pressure and water flow rate upstairs will be less than for downstairs.