Check the menu in the deck’s setup. I believe that you have the option of setting the deck to feed Ch1/2 audio onputs to Ch3/4 or to set up CH3/4 to have different, separate inputs.
I don’t have a deck in front of me right now, but it’s either a Menu setting or a switch on the slide out control panel.
Tom
Life moves pretty fast. If you don’t stop and look around once in a while, you could miss it.–Ferris Bueller