If you convert to the old master-local setup you can terminate AP's on both.
If you install a Mobility master, the 2 MC's will form a active active cluster so you can terminate AP's on both.
in both scenario's you can terminate 2x256 (you will loose redunancy)
other way is to add 3th local controller to extend the number of AP''s supported, this can be done via clustering under a MM setup, or add a local controller under your master/master-backup setup.
since you are investing in controller hardware i would advise to buy an additional MM so you can make use of all the good MM features including clustering.