Do not combine line audio without any isolation. And it doesn't matter if you haven't had any problems.http://www.mobileaudio.com/rac-faq/rac-faq_5.html#SEC106
"Creating a mono signal is often necessary when you are powering a subwoofer by bridging the amplifier. Many people do not realize that bridging an amplifier does not always provide a mono signal -- many amplifiers will simply use only one input channel, which means that the subwoofer won't be receiving the full signal.
Some amplifiers have a switch that will allow you to combine the left and right channels into a mono signal. Some signal processors and head units provide a subwoofer-out channel that can be switched between stereo and mono.
If you don't have this feature on any of your equipment, you will need to provide a mono signal to the amplifier. The common thought is to use a Y-adapter to "combine" the left and right channels. However, by using a Y-adapter, you are actually summing the line voltages and directly shorting the left and right channels at the head unit, which could cause problems.
The correct way to create a mono signal is to cut off the ends of the RCA cables, combine the signal grounds (the outer shield), and then use a 1 kOhm (1/4 watt, 5% tolerance) resistor to each of the center conductors. Solder and insulate the resistors so that you don't short them prematurely, and then connect the two resistors together. Connect the summed signal ground to the shield of the new RCA plug, and the summed center conductor to the center pin of the RCA plug."
Basically, your center pins need to be isolated by those resisters. I made a box long time ago using Radio Shack project box and RCA jack panel so I didn't need cut cables and it was easy to build. After you have a true mono signal, you can split it to dual mono.