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