That feature is (as far as I'm aware) usually called Reverse Stereo.
This webpage here has a suggestion. What it seems to consist of, is 2 short extension cables and two stereo splitters.Assuming you don't want to solder one up, you can do it with two 1/8th-inch-plug to twin-RCA-plug adapters, two RCA cable joiners (female sockets at both ends), and one 1/8th-inch cable joiner (female socket at both ends). Plug one adapter cable into the speaker output, attach an RCA joiner to each of its plugs, plug the RCA plugs of the other adapter cable into the joiners (with right going to left and left going to right; this is what swaps the sides), then attach the 1/8th inch joiner to the 1/8th inch plug on the end of the second adapter cable. Plug the headphones into the joiner.
All this isn't terribly elegant, but it's not too hideous or stunningly expensive, and it'll give you a few more metres of headphone cable, in case you need it.-Ordstrin (January 20, 2009, 08:55 PM)
Here's a simpler solution http://www.amazon.com/gp/registry/wishlist/3Y2NG30BLVMH/ref=cm_wl_rlist_go_o?
Or you can get these any larger 99c or dollar stores (or may be ebay)