Steph, you haven't given much detail about your setup, especially how far above the screen the center speaker is. If it's only a foot or so and isn't much different from the main speakers, then angling it down to point at your ears usually gives an acceptable effect. If this doesn't work for some reason, then yes, it's possible to run speakers in parallel above and below the screen, which will form a "phantom" image in the middle of the screen.
When speakers are run in parallel, this reduces the net impedance that the receiver sees. The simple answer is that two 8 ohm speakers in parallel reduce the impedance by half, i.e. a resulting 4ohm nominal impedance. The actual general formula is that the reciprocal of the total impedance resulting when any number of speakers of any impedance are run in parallel is equal to the sum of the reciprocals of the individual speaker impedances, e.g., 1/T=1/8+1/8, 1/T=2/8, T=4(ohms).
Whether any particular receiver would have any problem would depend on the impedances of the speakers, how loudly the material was being played and the capability of the receiver.
