You are only using 200watts. This isnt slaving.
You would have to take line out of amp 1 into input of amp 2 (with master at safest max setting) . Then both amps would each have to drive a 4ohm load from their respective speaker outs. You would control volume from amp 1.
This works better with amps that have an effects in and out so u can bypass the pre of the second amp.
Seems like you are just sending a hotter signal to amp 2 now.
Hmm I had assumed that was going on, but now thinking about it, those MB200s don't have returns!