Hello Djmatrix9113, Your question is somewhat lacking in critical information. In general, 2, 8 ohm 15" subs, wired in parallel present a 4 ohm load to the amplifier. Two of the same of any driver (size/make being equal) is twice as much air moved as a single woofer (same type) at any given constant wattage input. I assume you�re trying to get the most output overall so, figure the two 15's to be the better choice. Would need to know more. Why are you able to give a single 15� 1200 watts and only 600 to the dual 15 configuration? Me thinks you are caught up in the watts/output/impedance conundrum many neophytes endeavor to learn. If the amp is rated at 1200 watts at 4 ohms, then the 2, 8 ohm (in parallel, 4 ohms to the amp) 15's will put out twice as much as the same size/type single 4 ohm rated 15. This all assumes that the 15� drivers/woofers are exactly the same make and model except for the impedance rating. If the amp(s) and drivers are different, then it is a little (possibly a lot) more complicated. Steve