Three options:
1) hardware load balancer
2) Create two server groups. One with server A in slot 1, B in slot 2. Then the other, flip it. You can then flip between these server groups in your different AP groups. If you have multiple controllers, you could also just point one controller at A with B as a backup and flip it for the other controller.
3) starting in AOS 6.4, there is a RADIUS load balancing option in the server group settings.