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

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

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

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