2 bookshelves for a center

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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...