Dear,
Network Discovery is supposed to replace the on-prem scanner in the long term.
Combining both in the same network will not resolve the scanning issues you have.
If you want to keep using the on-prem scanner, we normally recommend installing a scanning server at each location.
The same goes for Network discovery, ideally you install a sensor in each location or vlan for security reasons or network segmentation.
Both scanners require the same access to the assets that they scan. The main difference is that the on-prem scanner needs to communicate with the database, while network discovery contacts the Cloud API directly.
In a transistion fase, both can work together, in the long end, it is best to make a choice which scanner to use.