I have a functioning system with a 1/2 HP shallow well pump and separate 20 gallon bladder tank--all new components working . I decided to add another 20 gallon tank to the same manifold as the first tank, but the added tank remains empty! The only difference is the initial tank uses a 1-1/4 inch flexible feed into the manifold and the added tank uses a 1 inch flexible feed. Any ideas why the added tank remains empty?

Yes. It is full of air, both above and below the bladder. As time goes by (weeks), the air below the bladder will dissolve into the water and the tank will fill. Be patient.

No, the pressure gauge gives no indication at all of how much water is in the tank. It only shows the "pushing force" of the air that is in the tank.

BTW, there is always at least some water in the tank. But when it is only a very few gallons, it will seem as though it is empty.