2 bookshelves for a 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.

.
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.
.

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!"