The AP6521 APs can only have one radio on at a time. From GUI you can clearly see under Wireless that only one radio can be selected at a time. The 1 in AP6521 refers to single radio.
There is no Interface radio 2 on this unit, you can either set interface radio 1 to 2.4GHz or 5GHz, but not both.
Thank you,
Chris
Christoph S.