The closer to a straight shot at the listener the better. You can angle the speaker upward and improve it if you put it on the bottom shelf, but if on the top shelf, the angle may end up being a lot less and thus not even look like it is "propped up".
As for getting a second center channel, that is usually considered a big "no, no" as the sound waves can cancel each other out or something like that. Look into it before trying it. In my first home theater, I has 2 center channels, but I had gotten a little circuit from an electronics guy at the time (this was a good 6+ years ago) that put a fraction of a second delay between the two speakers. It was quite odd, but seemed to do the trick in that the perceived sound was louder. Of course, I was doing a lot of "hacking" of my system to get a larger sound envelope around the listener. This was back when 5.1 was the only game in town and I had the electronics guy that I knew make me some boards that would "mix" various channels for me. For example, out of my 5.1 receiver, I was pushing the two center channels, the front right and left, the surround right and left, and then what I called the "front left-center and right-center" which were a mix of the center and either right or left channel into a seperate speaker for each of these new "mixed" channels, and I also mixed the two surround channels into two speakers that were at the back (today these would be similar to the 6th and 7th speakers in a 7.1 setup). So I was getting descrete 5.1, plus speakers inbetween then that were mixed with each input on a seperate volume control so that I could tweak it a little. It made for what seemed like at the time very good sound. 10 speakers, 2 subwoofers (one powered, and one passive under the seating platform), and Bass Shakers all around. By today's standards I am embarassed to even mention it.
So point of the long story is that look into the dual center channels as it may be more problems than it is worth.
Good luck!
