well.. this is the cable i use out of my volca. but if you have a stereo 1/4-1/8 already i don't see how this would be any different. you're absolutely sure you're using the tip side from volca1, and the ring side for volca2 to the Splitter connection before the patchbay?

yeah i couldn't find any mono 1/8 females.. which is why i linked this one earlier https://www.amazon.com/gp/product/B07PJVG2RC/

since he already has the female 1/4 mono splitter linked in the OP.