Bring on the criticism...

It doesn't matter if the sensivity of the two are different. The effect is the same a wall of sound. I did this to get as much sound as I could. Then I started to like soundstage and imaging. Now I use 1 pair at a time and my tube amps wouldn't exactly like the drop in resistance.
Although I still have some SS amps and Klipsch LaScalas which I have once or twice connected in parallel to vintage JBL C36 and C38s (Yes both using D-130s, 131s with 075 tweeters)They are exquisitely sensative and seem to compete well with the Klipsch. I do it just for sheer volume try playing BBBAD Bad To The Bone like that when you feel raucous.I am rally getting too old for that I must admit.

Hmmm. interesting.

The tonal balance probably becomes a bit warmer, as each side's output will combine in-phase at low frequencies, while as Shadorne noted you will get comb-filtering (partial cancellation) at higher frequencies.

Comb filtering often looks a lot worse than it sounds; apparently the ear-brain mechanism is relatively tolerant of it.

Not knowing the exact geometry of the setup I can only speculate, but it's possible that the output from the outer speakers is arriving late enough that directional cues from its output are suppressed by the precedence effect. If so, imaging could be holding up quite well. Timbre may be richer and more vivid as well.

Duke
dealer/manufacturer

I did this with a JBL Paragon and JBL L80T's on top!! I was reproducing rock concert volume and the sound was way better then either speakers by themselves.

http://sphotos.ak.fbcdn.net/hphotos-ak-ash2/hs081.ash2/37395_1391904432108_1067063525_31014705_7062778_n.jpg