I might be missing something in what you’re asking for, but surely to capture both mixer outs into one you’d just want a 2x mono > 1x stereo cable? https://www.vicsdrumshop.com/planet-waves-planet-waves-10-ft-stereo-1-4-to-dual.html
You obviously wouldn’t get a stereo effect given what it is going to, but at least you wouldn’t lose one side altogether.