I'm using a single KEF 201/2 as a vertical center. I see a lot of people use 5 identical vertical bookshelf in a 5.1/ 7.1 system.
I would not use 2 center speakers, though, because of the comb filtering.
I think horizontal centers were made because of space limitations, not for SQ.
So if you have the space, I would go with a vertical center.