hook both speakers up to one channel on the amp. 8ohms+8ohms=4ohms. that is the only safe way to do it. if you run the wrong ohms for any extended amount of time you will fry the amp... been their done that
ummm ya lookup wiring diagrams but if i can remember u should be able to drop the sub down to maybe four look up parallel n series wiring http://www.the12volt.com/caraudio/woofer_configurations.asp?Q=2&I=81