Several here like the single decent located in the center of the two CLS . If I had two I would try center and center behind me with each being 180* opposite phase. There is a report from Harmon International that says two subs that configuration is the best. That being said I would try corners side walls and anything else. You will be amazed at the different results. Sub integration is tough to say the least. With two its even more tricky. Start the x overs at 35 hz and volume at min and work up. with two subs you will get a few DB more than one at the same volume. Phase will be decided by placement. Good luck . Dont be afraid of a test disc to run a few graphs or REW software too.