Why combine them? Just use one and keep the other one packed away as the spare. I would also think combining them would tax your amp on the center channel side trying to power to many speakers.
When I used to run all matching sets as my HT set up, back in the day, I ran all the same speakers front, rear, and center, then just kept the last one in the box in case I blew one. Your also always better off having all the speakers exactly the same all around for if the sound changes, say a air plane, it doesnt go from left front from sounding like a plane to center sounding like a rc plane, back to right front sounding like a real plane again, to the rear going off in the distance sounding like its flying in a tin can. All the same speakers, with a sub to boot, and you get uniformity in everything you listen to be it explosions, to gun fire, to voices from all sides.
Just something to consider.