It doesn't need to be terribly complicated. A "duplex controller" does exactly this, using an alternating relay. But in a typical duplex controller for pumps or compressors, there is something that calls for the first machine, such as a float or pressure switch, then a second sensor that adds the 2nd one if the first one can't keep up. So if pump 1 fails, the level gets higher and automatically adds pump 2.
If this system only has one sensor, it's a little trickier, but not that bad. I used to do it with a n On-Delay timer and NC contacts of the starters feeding a relay that acts like the second sensor. The timer coil is in parallel with the alternator coil being controlled by the sensor. Then a NO delayed contact of the timer is in series with the NC aux contacts of the starters, then to the relay coil. A NO contact of the relay then goes around the alternator contact to each starter.
So when a pump is called for, whichever one is selected by the alternator comes on. At the same time the timer starts timing and a second later, the delayed contact closes. If the starter is running, the NC contact is open so the relay doesn't come on and only the one pump runs. If it fails and the sensor is still calling for a pump, its NC contact closes and the relay closes, which turns on the other pump.
Now I would do all of this in a Smart Relay or Micro PLC because it would cost less than the relay and timer and all that wiring, but not everyone is comfortable with that technology.