Adding an extra pressure tank will not increase the house pressure. Adding an additional pressure tank will actually make your system stay at low pressure for a longer time.

Pressure comes from the well via the pump, not from the pressure tank. The pump just sees the pressure tank as another demand it must satisfy. So the larger the tank, the longer the system will be at low pressure.

Water comes out of a pressure tank anytime a faucet is opened. However, not until the pressure has dropped from 60 to 40 PSI and the pressure tank is completely empty will the pressure switch start the pump. So you have no way of knowing if the tank is full to 60 PSI, or if most of the water in the tank has been used, and the pressure is sitting at 41 PSI when you turn on the shower. If the pressure happened to be at 60 PSI, then the shower pressure starts out high and decreases all the way to 40 PSI. Then the pump is started. The smaller the pump, larger the tank, and the more water you are using, the slower the tank fills as the pressure slowly climbs to 60 PSI before the pump is shut off.

The CSV will work with any size tank, and will still hold the pressure constant once the pump comes on and the pressure tank refills to the set point of the CSV. However, we still have to wait for the pressure tank to drain and refill before the CSV can go to work. So the larger the pressure tank, the more time it takes before the CSV can deliver strong constant pressure. With a small tank, like the 4.5 gallon size that only holds 1 gallon of water, the CSV is working and already holding constant pressure before you get the shower temperature adjusted.

Without a CSV, a large pressure tank continually fills and drains. The house only sees an “average” 50 PSI as the pump cycles itself to death going on and off between 40 and 60 PSI. The constant 50 PSI delivered by a CSV will greatly increase the pressure in the house as compared to an average 50 PSI.

Adding a CSV greatly increases the house pressure. Adding a larger or additional pressure tank decreases house pressure as the system will be at the low end pressure for much longer times, waiting on the large pressure tank to empty.