I havn't used the MIDI Megre yet as my A300 is my only keyboard. There was a user on this forum who had a problem trying to do what you're describing. You might need a dc power-pack to run both keyboards (the manual mentions something about this, i think.)
Having said that, i do know what you want to do is possible because i saw a video (sorry, can't remember where) of someone using an A-PRO as a master keyboard/controller and another hardware controller for the track's volume and pan controls.
Aswell as switching midi merge on at the side panel, you may have to adjust the midi merge destination on your A-PRO.
On your A-PRO, press the ACT button and the back button at the same time to open the settings.
Turn the value knob to the right untill it reads MIDI settings, then click the value knob. Turn the knob untill it says MIDI merge destination and click the value knob again. Turn the value knob to the right to select MIDI in. Finally, click the value knob again and it's all done.
Like i said i've never used thesse settings, but if you buy the A-800 PRO, can you let me know how you got on please because i'm still trying to learn about the A-PRO. Thanks.