A dual channel opto-isolator is two single-channel optoisolators in one package. Any optoisolator datasheet will show you how it is supposed to connect. Normally the input for a single channel terminal is two pins that is exactly the same as an LED inside the IC (so you need to add a current limiting resistor). THe output channel is a two more pins that represents the source/drain or collector/emitter of transistor that turns on when the LED is lit up and drives the transistor gate.