A:Answer I also have a Denon 7.1 unit with 2 sub woofer outs.
I have 2 Polk Audio 10 inch sub woofers that uses the two RCA inputs (1 right and 1 left) The cable your looking for has one RCA plug on one end and two RCA plugs on the other end. The end of the cable with the one RCA plug, plugs into your Denon sub out- the end that has the two RCA plugs, plug into the sub woofer RCA ins.
KabelDirekt (10 feet) 1 x RCA Male to 2 x RCA Male Subwoofer / Y - RCA / Digital Audio Cable - PRO Series