You could try doing an AP specific profile for the AM AP. Put it in AP mode, and enable Mode-Aware ARM on just that AP. If the coverage is dense enough, it should convert itself into an AM and only change back if necessary (if the other AP fails).
You will need to test this out after hours at the location. Try what I said, then try disabling the normal AP. See if the Mode-Aware enabled AP turns itself back into an AP (assuming the coverage was strong enough for it to become an AM in the first place).
Are there only two APs at this site?