@droftarts Thank Ian, this gives me enough to find a solution! Since the wiring (of 2 dual switching hotends) is still under construction, it might take a while till testing. I'll report results. Maybe there are more lunatics to try printing with 4 hotends like this .....

Hi. Trying to connect a Dual Switching hotend v2.1 (Makertech) to a Duet3 6HC gives me issues. The change of hotends is managed by a servo. This 5V servo is advised (by Makertech) to go by the accompanied relay switch. This results in 6 wires, of which 2 are 'signal'. Connecting to the PWM as suggested is straightforward. The PWM should be combined with the Io-9, with only 2 wires.
This is beyond me, but critical for the workhorse I'm constructing.
Has anyone suggestions to tackle this?
Appreciated.