Hi JD,
I'm not sure if it's officially documented as 'supported' anywhere, but I just setup a test in my lab using 2 ISE servers and I can successfully authenticate to the ISE GUI via a second external ISE server. My setup is [ISE 2.7] <=> [ISE 2.6].
My 'ise27' is configured as a RADIUS client in 'ise26' and the necessary Policy Set, AuthC and AuthZ Policies are configured to simply return an ACCESS-ACCEPT result. I'm using an Internal User to test, but it should work with an external ID store as well.
In 'ise27' I configured 'ise26' as a RADIUS Token server and configured the Admin Access to use 'ise26' for Authentication.
As with the OTP use case, ISE can only use internal authorisation, so you'll have to create shadow (External) user accounts in ISE for any RADIUS users that will need to connect to the ISE GUI.
Cheers,
Greg