Hi
I don't know if you can prioritize the AP towards 2 controllers, but it is good practice to create AP system profiles pointing to the controller it should connect.
In this LMS field you could specify master and backup controllers to which the AP is to connect to.
If you make 2 profiles, one for the pri 1 AP's and one profile for the pri 2 AP's, you could place these profiles on 2 identical AP groups to accomplish what you are looking for.
When the master fails, only the ones with a backup LMS IP configured will be up on the controller with the lower capacity.
Roar Fossen