Agreed with Tuomari on unclear what you are trying to do.
That the input (aux) is RCA has nothing to do with anything, so bringing them up only confuses the issue.
Then there are master outputs in both RCA and balanced (TRS) jack.
If you are trying to get the red and grey end of the Y-cable into the controller, that is not gonna work. A TRS cable does not carry a stereo signal, but a balanced mono signal. So at the single end of the Y-cable you can never get a stereo signal on the jack.
Please tell us what you are trying to hook up to what and why and we can try to give you the proper advice. Best advice for now is to stop using the cable as using the wrong cables can lead to connecting things that are not supposed to be connected, like “shorting” your L + R master outputs!
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