Hello FlyBy,
If I understand the concept, you would have to create one WLAN with RADIUS AAA, one WLAN with Captive Portal and bind them to relevant radios (radio 1 or radio 2).
For the sake of curiosity, what is the use case? There is also an option to have RADIUS AAA with fallback to Captive Portal if primary authentication fails.
Hope that helps,
Tomasz