Can I use both Endpoint Policy ManagerOn Premise mode and Endpoint Policy Manager Cloud simultaneously? Do they clash?

Netwrix Endpoint Policy Manager (formerly PolicyPak) On-Prem Suite and Endpoint Policy Manager Cloud have been designed to play nicely together.

Therefore, you can use either the Endpoint Policy Manager Cloud to deliver your setting and/or, say, Group Policy or SCCM to deliver your setting.

All policies are simply merged together. If there's a conflict, the on-premise directive (say, using Group Policy) wins.