You CAN, but I don't know why you would really want to. Just use one of the speakers and it should do the job fine.
If I remember this correctly and depending upon how you wire it (in tandem or in parallel) you will be cutting the ohm rating in half for the speaker(s), thus demanding more power from the receiver/amp to get the same current and therefore, volume as the rest of your speakers. You are better off doing as I suggested above or just using your front left and right speakers as "phantom" centers, if you want two speakers to do the job as a center.