A lot of people run a Y-splitter the other way to change a single LFE out into L/R output for subs with two inputs. It sometimes helps with subs set to automatically turn on.
What you want to do (combine two L/R RCA signals into a single one) isn't as straightforward as I understand it.
I don't know enough about it though to really be helpful.
If you do get a simple cable that will go from L/R to a single RCA connection, it will probably work just fine. There may be issues with overall signal level and interference of the signals though. I don't really know how subs with L/R inputs change the signal into mono for output though, which is what you're trying to do external to the sub.
Random google search results
http://stason.org/TULARC/entertainm...rn-a-stereo-signal-into-a-mono-signal-BW.html