Searching for a way to combine stereo channels for my FM mono transmitter, I found this article back in 1996. I had a link to this information in my favorites for years until it finally came up "404." Thanks to the Internet Archive I was able to recover this seminal missive and present it here for you.
I designed Bus’tr simply as an aluminum project box with 1/4” circuit jacks and 680 ohm build-out resistors. To use it, you just hook 2 outputs of anything you want to mix together into 2 of the jacks and the signal comes out the 3rd jack, simple as that.
Notice that all 3 jacks have a build-out resistor, allowing any two jacks to act as an input. You can substitute RCA jack for the 1/4” jacks if desired. For this configuration, be sure that the equipment you’re hooking together is rated at 600 ohm or less output impedance. Higher or lower impedances can be made to work by substituting different resistor values
I have also used lower resistor values when the source output voltage was higher than a line-out connection.
See the complete article by Mike Sokol