How to proxy some requests to another Radius server in the same realm?

[little-nemo at virgilio.it]

Our freeradius 3.2.1 for Eduroam (Radius2) receives requests from users in Active Directory (for example, SSID=staff-AD).
We already have a primary Radius server (Radius1, different system) which usually handles the students' and staff requests (for example: SSID=students), which credentials are stored into the internal database.
 
How can I forward the requests arriving to freeradius from students (who tried to use SSID=staff-AD, being rejected by AD), forwarding them to Radius1?
How can I forward the requests arriving to freeradius from SSID=students, forwarding them to Radius1?
 
When I think "forward" I guess use "proxy.conf" but when I think about authorization (rejected by AD) I guess I should act elsewhere (default?).
Are there examples to study and adapt? My previous searches on the documentation and the code let me confused.
 
Thanks in advance.
 
Pietro