@Søren-Heide-Larsen different networks can be achieved by setting different channels / network ID.
See here for NRF24 and here for RFM69.
There is no pairing function in MySensors.
@GLAB No worries, was in a similar situation.. My perspective was on what the Node could be made to provide, not what could be done within Domoticz to address the requirement - eg Had heard of Dummy counters but had no experience nor understood their purpose, nor had ever used scripts.
I had been sending a logical ON/OFF as well as a cumulative total from the Node and hit the same hair-pulling scenario as yourself.
Now only the ON/OFF is sent and the short script makes the transposition within Domoticz to a dummy meter (I think kWhr) using the internal clock in Domoticz.
Have fun