Based on this Configuration Guide, it would appear to only support one portal. I don't know if there's a way to specify the SSID (network) to the portal page / WLC. (I've never needed to. [it's in the `wlan` query param])
Update: Apparently 7.3+ can, but it's a lot more involved. "Assigning Login, Login Failure, and Logout Pages per WLAN"