Both decks through one channel

Hi
I've seen that people have had this problem before but I havent found a solution.
Both decks are playing out of channel 01. when I change the second one to 02 I get no sound.
I had this problem initially and it rectified - though i'm not sure how.
It has been working fine for a few months and now it suddenly has reverted back.
Help please.
I've seen that people have had this problem before but I havent found a solution.
Both decks are playing out of channel 01. when I change the second one to 02 I get no sound.
I had this problem initially and it rectified - though i'm not sure how.
It has been working fine for a few months and now it suddenly has reverted back.
Help please.