Like always, whenever you want to do "just this" it turns out all bunch of things get in the way.
It turns out things can't really happen as I imagined. Here I'm thinking about the blend between the bass guitar sound and synth sound. Using just a double potentiometer that was in the guitar wasn't good since the clean bass sound is needed as an input to synth (main out was "leaking" back to synth input which caused feedback). That's why I added a simplest JFET buffer before the blend control. I also added the same buffer to ADC input but with a bit higher gain (around 2).
TODO: add video and some description