That's actually part of the initial problem that leads me to consider 2 side by side bookshelves ( slightly toe out ) vs. 1 center on end.
The center in the vertical plane will deliver better than the the center in the horizontal plane, but then it starts to intrude on the TV - OTOH, the tweeter on the center will be on the same plane as the woofers, if it's horizontal - and I was thinking that the side by side bookshelves, would at least keep the tweeters out of the same horizontal plane as the woofers and be shorter that the center on end.
No.
Sure, not having the tweeter on the same horiz plane as the woofer would be ideal. But, this is not even close to the biggest issue here.
Firstly, on the typical horiz mtm center, it's that the TWO WOOFERS are playing the EXACT same stuff, WHILE in the same horiz plane. This is what creates
significant lobing in the horiz plane.
Now, two bookshelves, vertically, side by side, playing the same stuff, not only gives you lobing between the woofers, but now you also have lobing between the tweeters!
The extent of lobing, at least to the extent that the human can easily detect, will depend on both the xover point between tweeter and woofers, as well as the distance between the woofers. The closer these woofers are, the less offensive lobing will be.
You've got it all wrong. My advice would be: instead of thinking of two speakers, think of one single better speaker. Yes, there are some bookshelves that are so bad out there, that a horiz mtm still sounds better, given the typical angle of listener to the center speaker. Concentrate on finding good speakers, first of all. That is my 2 cents.