selective combination of channels

This is my first post, but I’ve looked around these forms before and found answers to my many questions.

I want to mix L and R channels for my homemade oscilloscope without mixing them for the speakers. Once the 2 channels are combined inside the device both my speakers play the same noise. I knew this would happen when I did it, however i expected finding a schematic to solve the problem would be easier. My problem is I don't know what to search for.

Thanks for any help in advance. attached is an image of the creation, shutter speed it too slow though ;)
 

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