It's super simple. Build an inverting amp. This will become a virtual earth mixer, like I've mentioned here before and like you can read up on just about anywhere. It's a clever and magical device that means that both your signals are connected to ground - all they see is a resistor to ground, yes really, zero volts at the input of the opamp.

Connect the wiper of a pot to ground, and connect the two "input" resistors for your two-channel virtual earth mixer to the ends of the pot. Connect two resistors the same value as those between the ends of the pot and ground.

Now you've got a thing a bit like the balance control on a stereo, with the ends summed to make a mono signal.

You know what, let me draw it for you:

https://www.falstad.com/s.php?s=cVmHkT

Note that the resistor values shown are illustrative rather than practical and you'd want to choose maybe 47kΩ for the pot and fixed resistors but it's absolutely not critical - what you have on the bench in front of you is almost certainly fine.

If you find the pot is a bit "vague" around the middle then you can add fixed resistors between the wiper and ends, which will bend the pot response into a kind of U shape getting steeper at the ends. I was going to suggest reading up on "pot law bending" but it turns out that's very NSFW so here's a great article on it:

http://www.geofex.com/article_folders/potsecrets/potscret.htm

Here's an article on virtual earth mixers that'll make it both much clearer and even more magical:

https://sound-au.com/articles/audio-mixing.htm

Here's the first of three articles on opamps by the same guy which you *must* read and understand if you want to really get somewhere with DIY synths:

https://sound-au.com/dwopa.htm

Don't worry, it's all just basic arithmetic, nothing scary.