The output will more or less be the same (both speakers are powered by the same amp; the VP150 can take 400W so the speaker is definitely not the limiting factor in your application).
The advantage you will get is improved "anchoring" of the dialog on the middle of the screen. This can improve hearing of dialog simply by making lips movement and sound direction more realistic for your ear/eye coordination (do not underestimate how much the brain works and uses feedback to improve hearing).
I have 2 VP150 and it helped quite a bit (always felt like the room was a bit skewed whether I put one on top or bottom).
