if you had a mono amp, and the sub out is a single cable, yea it'd just be one channel (connector)
if it's going into an amp that takes a left & a right, just get a rca splitter.
spliced @ head unit:
get a splitter that's 1male and 2 female. hook the male end in the head unit, and then hook the long (stereo) rca cable (male both sides) into the 2females of the splitter
@ amp:
get a long mono rca cable (basically just pull apart a stereo rca cable and use one of the wires) (male on both sides); hook that into the head unit, then at the other end, get a 1female and 2 male splitter