A NPS can be configured as a proxy server, forwarding request for radius authentication to other RADIUS servers.
To forward request to other RADIUS server, under Connection Request Policy. Open the policy and on the Settings Tab click on Authentication and then New
Enter in a Policy name and Add RADIUS
In the Add RADIUS server, Address tab enter in the name of the RADIUS server.
The Authentication/Accounting tab allow for entering the Shared Secret and specifying port numbers.
Priority and weight setting, enables load balancing for the group. The larger the weight, the more requests will be sent to the server.