I can't speak specifically of AAon, but I worked on a new Trane 8.5 ton plenum fan style RTU with dehum. It's logic runs both stages of cooling, and energizes a 4 way valve for reheat on a call for dehum. The reheat coil is connected only to 1st stage compressor. There's no way to run only 1st stage comp during dehum on that unit. Even with both stages running it yields a 4* delta t. Hope this info helps some. Maybe your 2nd stage only starts when the temp rises enough in the space to call for cooling? Have you checked the charge? Make sure your hot gas bypass isn't messing you up. I;m guessing that should only be active during low ambient conditions.