Connect a second NDES server to enterprise CA

In preparation of the migration of our ~4500 MDM-devices from SCCM-Intune-Hybrid to Intune SA, I am trying to install a second NDES-server to be able to test and provide feature-parity before moving the first users.

Anyone has an idea how to do so? 

Always get an error (during NDES setup wizard) stating, that the "endpoint is a duplicate".

The same error can be found on the CA in the eventlog, but in my opinion, it should be able to connect to a CA from two servers, even as I use different system-users and also created a new cert-template for this purpos.

Any thoughts on this are appreciated

Thanks in advance

Julius