The video is incorrect. It is simple physics. The water and the air in the tank will be the same pressure. Adding a second tank does not double the pressure, this simply goes against the laws of physics. Just like increasing the air pressure in the tank does not increase the water pressure (except for the water that is currently in the tank due to the temporary differential pressure), it only takes away water volume. You will have higher pressure at the end of the tanks water volume, but you will do so at the sacrifice of water volume... sorry, physics 101 lesson here.
The only way you get more volume due to larger or more tanks is the pressure stays higher longer due to the amount of compressed air in the tank... the more tank volume, the longer the pressure stays higher.