You are going to use a "Y" cord...(link)http://www.amazon.com/C2G-Cables-03177-Series-Y-Cable/dp/B0002J1LE8/ref=sr_1_27?ie=UTF8&qid=1342709209&sr=8-27&keywords=RCA+Y+cord
Two female RCA connectors -to- One RCA male connector
The male connector plugs into the sub output at the receiver...
The two female connect to your stereo RCA patch cord, to the sub inputs.
Available at radio shack or other electronics store, or order from internet...
As far as "monster" cable, or other overpriced cable, it's a complete waste of money. It accomplishes nothing.
Inexpensive cables, from RS or Walmart, have gold plated RCA plugs, Will work just as good as any expensive cable. There is no difference in the sound or performance.