You can get RCA Y adapters, might be easier than splicing. Bear in mind you won't have control of front/rear balance (if that bothers you at all?)
This should be what you need:
http://www.amazon.co.uk/Splitter-Lead-Female-Male-Pair/dp/B004T8ZAPY/ref=sr_1_23?ie=UTF8&qid=1359219610&sr=8-23