1 RCA output as in 1 left plug and 1 right plug?
There is a left and a right on the amp, a set of RCA cables consists of 2 wires and 4 plugs. 1 plug at each end of each wire which makes 2 plugs per end.
2 plug into the back of the HU, 1 in the left hole and the other in the right hole. And...