depending on how you wire the speakers, (parallel or series) it will affect the resistance that the receiver sees. The power each speaker sees will be cut in half... just get another cheap amp to run the 2nd set...using the pre-am,p from the reciever to send the same signal to the back
If in doubt, wire in series. I'd wire series without a second thought. Your amp will easily drive the 12 - 16 ohm load and the little bit of volume correction can be made with the amplifier's settings.
Yeah you can do this, if in parallel the impedance drops. IF you have a competent receiver it shouldn't be a problem unless your rears are already low impedence speakers. I've done this before with no problem. Especially with rear backs there's usually not much going on back there anyway.
But you didn't mention the receiver or the speakers, so hard to say...
The receiver is a kenwood VR 7080
The speakers in question Yamaha NS-5290. That is if I can find 2 more to match my existing 2. If I can't find em (so far I cant) then I'll buy 4 of something new. What kind of speakers would I look for knowing what I want to do with them?