Yes, you can define multiple RADIUS servers per Zone or per WLAN in a Zone.
Ruckus SZ supports 2 ways to authenticate with RADIUS:
1. Direct AP to RADIUS - AP's much be able to reach the RADIUS and each AP will be a RADIUS Client (NAS with permission to consult RADIUS). This is configured per AP ZONE and then referenced per WLAN in that Zone.
2. Proxy - Where AP (still acting as NAS) sends all RADIUS requests in the AP control tunnel to the SZ where a RADIUS proxy then consults the RADIUS server. Big advantage of this is you only need to define the SZ nodes as RADIUS Clients (not to be confused with RADIUS user data base). All RADIUS traffic will go out the Management IP (if using 3 Network configuration vSZ-H). I believe you can define up to 32 RADIUS authentication profiles in a Cluster. RADIUS proxy profiles are selected via a RADIUS Realm Authentication service (permitting the same RADIUS profile to use Real/domain based selection) to the WLAN configuration per Zone.
I hope this answers your question.