I hope I found a solution:
The pressure switch switches at present directly the well pump.
The idea is that it will turn on 4-phase impulse switching relay.
To this relay both pumps are connected (4-wire, 380V, each connected as a group).
In addition, this relay is connected via the 24V control line directly to the Opensprinkler.
For the OS, the relay is an irrigation section, which e.g. is turned off on even days, and on on odd days.
If the pressure switch now detects a vacuum the current is switched through to the switching relay.
The relay drops the stream then passed to the respective pump.
If the pressure switch detects no vacuum, the relays is without electricity.
In this case (it´s a failure of the supply voltage for the relay) the switch power off defined.
Here, the circuit state could be changed.
But next time the relay is switched on, the OS control would work again and switch to the correct circuit/pump.
The impulse switching relay is a Eltako ESR12Z-4DX-U