Well the dual out should be a front/rear out of RCA's having two white and two red RCA outs. The Subwoofer out should have one red and one white RCA out, just run one set of RCA's from the sub out to the line in on your amp. peace
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
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