> line array
A Line Array has infinite horizontal dispersion; or pretty-much. This is a way to avoid the radical changes of dispersion found in simple cones and horns.
In practice, a "line array" for PA will be wide in bass and worked up until horizontal dispersion narrows past 90 degrees; or have semi-horns to hold 60 degrees from mid-bass to crossover.
A Ten Inch cone (8" working diaphragm) will be about 60 degrees at 1,700Hz. This is not out of line with the claimed 2KHz of your proposed ribbon (which I doubt will survive high-level club work).
But your 21"-wide duplex will focus to 50 deg by 650Hz. And will have very erratic off-axis lobing from 500Hz all the way up. In particular it will have severe side-lobes at 940Hz, where the two cone-centers are one wavelength apart, and reinforce each other in the typical direction of the microphones.
In typical clubs/dives, only slight horizontal directionality is needed; the audience sits all around the stage; but huge vertical directivity is useful to control reverberation. And it is a sour cliche to be directional 350Hz up (15" cone on box) and all boom/mud omnidirectivity in the 150-350Hz range.
BTW: that wide-pair scheme was designed for home theater, to "project" the midrange of the center (dialog) track to the sweet spot (the owner) without wall-spray. In a club, this is just wrong: folks on-line get too-strong mids, folks everywhere else get weak sliced-up mids.
You want your array over 4 feet tall, but as narrow as possible.
However for high acoustic efficiency you want 16 square feet of radiator.
There's no 100% way to resolve the two goals.
Well-designed line arrays often use 6" cones up to horn crossover. However a good loud strong Six is expensive and you need a lot of them. The low price of commodity gitar speakers is attractive.
Set your Tens just one-wide, and as many up as you can fit in the Econoline. A 1*8 stack of Tens is a very impressive sound-thrower in a low room, and pretty effective outdoors. It does get beamy above 1KHz-2KHz.
The range above 1KHz is where line-arrays are really hard to do well at low cost. In a project for a very long cathedral I've sketched four Eights. The back wall is only 20 degrees wide from the speaker location, so this will smear 10KHz all across the back; however it may be quite dull at the ends of the front pew and at the podium.
I have not tried these, and I hate piezos, but at a buck each, you can array three to each Ten and get your shrillness cheap.
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=272-306
If you can cover <200Hz some other way, a one-wide tall-stack of Eminence Alpha-6A will give very smooth sound across the 200Hz-3KHz range, keeping most of your "voice" in one bandpass rather than sliced by crossovers. Past there, an array of cheap piezos is as good as anything after the second beer.
Pyle Pro PDMR5 or Dayton PK165 are much cheaper alternatives, and will sizzle pretty good over most of the place, leaving less for piezo fill.
You don't want my advice. I spent a lot of my youth exploring "better" PA systems. Did some good stuff. Also had a lot of push-back from performers who did not really want something different than what they grew up on. Or, by tuning for performer's on-stage expectation, gave the audience bad sound. And always a lot of work building and refining, re-building after road accidents. Adopters re-jiggering my careful plans all out of shape. Hundreds of hours of work with not a whole lot to show for it. I do think the better factories, like Peavey, DO give you excellent value, with a wide range of "how good can you afford?" Once in a while I hear one that is jaw-dropping, 98% as fine as my RCA drivers on hand-carved wood horns, ready-built and warranteed.
Get another Peavey, Mackie, JBL system. Especially this year, pawn-shops and other sources must be flush with gear idled by the economic downturn, like the Porsche for-sale on the lawn across the street.