I'm not sure from the description what the relative locations of the pressure switch and tanks are, but here is what you MUST have.
1. Both pressure tanks must be connected so there is no check valve that would prevent water from either or both pressure tanks from supplying the system. Both tanks must always have the same pressure because they must be connected without an interfering check valve.
2. The pressure switch must be installed so that both tanks are connected to the pressure switch without any interfering check valve. There must be no check valve that prevents either tank from pressurizing the pressure switch.
If conditions 1 and 2 are met the pressure switch, pressure gauge, and both tanks will be at the pressure going into the system. I saw a system with six pressure tanks connected via a manifold arrangement and they worked fine.