joining two rca wires to one?

i have a subwoofer that takes a single mono RCA in.

coming from RCA stereo left and right output on a sound card.

wondering if there is an easy(and cheap) way to do this.

i fear simply using a splitter in reverse will backfeed the signal to the amps and fry the soundcard outputs over time.

Quote from alavux;31155

http://www.rane.com/note110.html

that's a nice guide, but i don't see a solution to the problem there

i'm taking two signals with this:

and joining them to this:

k so any signal coming from one channel back-fed into the opposite channel won't cause any problem to them?

if i can use a splitter(which i already have) that would be perfect. i know it works, i just wanted to know if it would cause any long term damage. i just would like to take every precaution to protect my equipment because buying new stuff can get pricey.