switch for two tank pumps control

I have a small greenhouse with a two tank cistern--graywater and rainwater with a pump in each tank--and need to switch automatically from the graywater pump when it is nearly empty to the rainwater pump. All I've been able to find are on/off switches; what I need is the equivalent of a three way electrical switch from one circuit to another.

Does such an animal exist? Any help appreciated.

If a moderator is here, will I be emailed if someone replies? Or do I have to keep checking? Thanks.

If your pumps are 120V, a simple 3-way (SPDT) light switch would do it. Common as borscht.

For 240V you would need a DPDT (double pole, double throw). Harder to find.

Thanks. Yes, 120V. That was my first thought, and would be a good solution to do it manually. However, I need it to function on it's own. The circuit is on a timer to control the total amount of water pumped. I need a way to check and empty the graywater tank first, then switch to the rainwater for the rest of the time. When more graywater comes in, it resets to that pump. I thought of making a mechanical Rube Goldberg design using a 3 way switch and a float valve, but this is sketchy. I checked with the plumbing shops locally, and they knew of no such switch.

Sorry, I didn't get that you wanted "automatic". I see you updated your OP since.

I think you are not likely to find a canned package and will have to make your own from a float switch and a simple DPDT relay. A float switch in the grey water tank as used for a sump pump would work to energize the relay and pump. When the switch opens, the relay falls out and "flops" to the other pump.

A flip flop (AKA latching) version would need energy from one source to flip it and energy from another source to flop it so two floats would be needed; a low limit and a high limit.