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Troubleshooting 101:
Change the cable out of the CDJ-700 connected to channel 2. If the problem went away, the issue was the cable.
Swap the left / right connections on ONE END for the CDJ-700 on channel 2; did the audio change to now only come out of the left side? If it swapped sides, the problem is possibly the CDJ output. If not, try below:
Swap the input from the CDJ connected to channel 1, connect it to channel 2's inputs. Does the problem persist on channel 2? If yes, the problem is the mixer. If the problem is now on channel 1, the problem is definitely the CDJ output.