I am also running a 2 speaker center channel with my 990.

http://tinyurl.com/en4ml

Speakers are wired in parallel; simply piggy-backed the banana plugs from one speaker to the other. Reasons for doing this were:

1) I did not have room to put my speaker(s) vertically under the (100") screen. With 2 speakers, I maintain system symmetry with the speakers on their sides.
2) I had the spare speaker; so why not? What else was I going to do with it? ;-)

As others have stated- doing this will half the impedance seen by the amp; if you're using an 8ohm speaker, 2 speakers in parallel will present the amp with a 4ohm load. This may or may not be a problem for your amp(s). The alternative is to split the center channel source signal from the 990, and feed a stereo amp, driving the spekers in syncronous "stereo". I personally think the mono-amp, parallel speaker route is a cleaner way to go.