2 center channel OR 2 bookshelf?

Bookshelves would be better,

- vertical speakers are better

- MTM of centre channels is a compromise for height and has lobing issues

My understanding is if you have two centers doing the work of one, you might indeed get more volume howver, you will have two sources putting out the same sound which will interefere with each other in a destructive way.

You would perhaps be better off to get a single, larger, more sensative center.

Better yet....if you have the space, get three of the same speaker and have three identical across the front and eliminate the problem entirely.

That really does not make much sense; the speakers you are talking about are not designed to be used that way and are not going to perform well.

I would suggest that you consider a pair of the PSB Synchrony One B speakers.

They would sound like a stereo pair of those specific speakers turned vertical. You have given no info on the specific center ch speaker (it's driver layout, LF alignment, etc), so no specifics can be suggested for the "sound", only the above WAGs.
This often yields significant bass reinforcement, but possible holes as well, depending upon proximity to other boundaries. Again, no specifics yields...no specifics.
See above.

cheers,

AJ