Automatic water valve fitting

[Jakebeckett]

I have an underground water tank with ball valve to control its level and whose water is pumped to a tank on the roof. Each tank has a float switch, these control the pump and the water levels. I have just built another underground tank. My question is how do I set up an automatic system so that if one underground tank becomes low the pump is switched to the other tank and vice versa if that comes low. Phew, hope that makes sense. Tks in advance