Stinger makes a 4 channel RCA cable... it combines 2 - 2 channel cables into a single cable, then splits at the ends... really nice cable... I have one in my car...

You should be able to go shorter than 20ft... they usually are measured in meters for some stupid reason (yes, I saw your post about meters in A/V)

Shorter cables, means less resistance for the audio signal... meaning a possible cleaner signal and less chance for noise to get in the signal. This is a big help for decks with 2 volt or less pre-amp outputs.

A good idea is to use a piece of string, run it where the cable is to be run... measure the string and add a foot or 2 for error. Keeps from buying cables too long or too short.