2 bookshelves for a center

I use to have a pre that had 2 center outs for that purpose.

I've been doing this for some time. I had an extra Soliloquy Sat-5 (one for the center and one for a proposed rear center). I scrapped the rear center idea and put the pair together in the front. With a big projection screen, it helped widen the soundstage some and alleviated the stress on the single speaker. I have a 6 channel Mac powering my system using a splitter on my center out.
The sound is great. Lots of extra headroom. I have the centers laying on their sides in a general MTTM configuration. It works way better than a single center.

I advise against it. The standard MTM center is bad enough in terms of HF interference and corrupted radiation patterns. Using two speakers side-by-side, whether horizontal or vertical, would increase these problems. (Have you noticed that virtually ALL quality speakers have their drivers arranged vertically? )

A single vertically-oriented speaker in the center would provide more consistent and accurate imaging, to say nothing of better timbre-matching.

Kal

I agree with Kal. I tried two VR-1 bookshelf speakers as center speakers and couldn't get it to sound right. I bought a VSA center speaker and suddenly it disappeared. I could always tell the center bookshelf speakers were there.

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If you can get the Usher's on a trial basis....you have to try it...it may work. I've seen threads here where a few people have tried it and loved it. There is no consensus on this, so give it a try if you get the opportunity.
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There may be no unanimity but there is a consensus based on experience and science. Now, of course, it is always best to try something that interests you (as long as it does not cost a limb) and decide for yourself.

Kal

Actually, if anything - especially for a relative novice (which most of us are, let's face it)- if you were going to go with two center speakers, I would suggest your likely best results with that arrangement would be had if you had the two center speakers placed rather far apart (possibly spreading them like having 4 (L/C/C/R) abreast across the room, or maybe having one high up, say 2 feet from an 8' ceiling and one down lower, say 2 feet from the floor - which would have you locating the dominating sound from the lower placed speaker). This would help reduce critical range comb-filtering issues likely to pop up with two speaker playing mono information next to each other.
The down-side of a wide spread side-by-side setup for a center though would be dialog location shifted to one side of the sound-stage. So, in that regard I suggest the one high/one low setup for dual's.
The benefit from using dual center channels, if any, would come from increased dynamic range and potential reinforcement from having more drivers in the mix! You would get increased efficiency, and more solid reinforcement of sound potentially (as long as you're not dealing with phase cancellation, as mentioned).
Still, I think having a "BETTER center speaker" is usually going to be the better answer! Easier to get sounding good, I think. Just make sure you have a good solid speaker as that anchor to begin with.
But, the audiophile in me likes to tinker too. So for that I say to you "good luck!"

Separating them farther does not eliminate the interference, it simply extends it down into lower frequencies (frequencies whose 1/2 wavelength is less than the distance between the drivers).

Your suggestion of using them one above and one below (both aimed at the listener) is a good one if one must use two. This would have the interference patterns confined mostly to the vertical plane giving the listeners in the seating plane a better soundfield and some ability to move around.

"Still, I think having a "BETTER center speaker" is usually going to be the better answer!" Ditto.

Kal

"Separating them farther does not eliminate the interference, it simply extends it down into lower frequencies (frequencies whose 1/2 wavelength is less than the distance between the drivers)." (kr4)

Yes, indeed. However, running the center channel as "small" or 80hz region should help allow one to keep the speakers a reasonable enough distance apart, that there shouldn't be a problem. I would think that affecting the upper bass/mid frequencies would be the critical range to not be "filtering" but I could be wrong here. (as bass is very important to remain intact, as much as possible, I understand)

I guess I should do the math, just to know that range if ever needed.
Lol - yes, better center sounds better to me too...

Do the math. Remember, there will be interference of all frequencies ABOVE the one whose 1/2 wavelength equals the separation. Therefore, a low cut is irrelevant.

Kal

Kr4, really? maybe my thinking is backwards. Because from what you're invering, if you place the speakers, say 100 feet appart, than ALL the frequencies are filtered between the speakers, and you'd be canceling out frequencies across the board - as you stood in front of one speaker, or closer in proximity, etc!! So, this can't be right, can it?
My understanding is that any frequencies, who's 1/2 wavelength distance between the two drivers playing the same mono info between them, was LESS than the 1/2 wave length distance, then you'd run into some phase cancellation issues in that case (so, for instance, basically the tweeters would only cancel each other in certain frequencies it covered if both tweeters were too close to each other, and you got off axis on one or both - like in a THX speaker arrangement - and your saying any mono info between two tweeters spread apart will comb, yes?).
Am I looking at this wrong? Because if that's the case, then what you're saying is that most ANY L/R speaker stereo setup, where you listen off-axis, would be filtering frequencies across a large portion of the frequency range!!!
(which should make the sound "horrible if you sat off axis)
Well I'll definitely do some research now, either way - lol