Yes absolutely you can load balance the access points between controller as long as AP can talk to controller and find the master.
You can configure LMS IP and Bkp-LMS ip from AP system profile on the controller ap-group.
LMS-IP -- this would be primary ip address of the master controller where AP would try to talk to and terminate GRE traffic
Bkp-LMS IP -- this would be secondary basically fail over mode when primary is not reachable or in failure.
Sriram