2 bookshelves for a center

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)." (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...

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