1) Set both GPOADmin servers to use server and port SPN format, as outlined under "To configure GPOADmin to use multiple service accounts" on p21 of the 5.16 Quick Start Guide. This is required to have this setup work as the customer expects.
2) The service account for both GPOADmin servers must be able to create service connection points for the GPOADmin services. See p17 step 18 in the GPOADmin Quick Start Guide
3) The new GPOADmin service should be restarted once (1) and (2) have been verified. Check the GPOADmin service logs once the service is restarted and if the service account has any issues setting up it's SPN or SCP the failures will be logged in the event log with steps to correct the issue in the log as well.
Once these steps have been done attempt the GPO creation again.