The sub is going to push the 6x9's the wrong way and distort them like crazy...but still cool idea.
I made a 'death speaker' in college out of 2 sheets of mdf...had a couple 15s, couple 12's, couple tweeters, etc in it. When the subs really got going, they would blow everything else. We ended up making a wall between subs and speakers and it fixed most of it. Think someone in the frat still has it.