Here’s a separate discussion (and feature request) on that topic. Currently there’s not a way to use stereo “Y” cables in the QC’s FX Loops.
Ultimately it may come down to the hardware level. If each FX Loop only has a single set of DAC / ADC converters, then they’re forever mono. If each FX Loop has a dual set of these converters, then theoretically each of those jacks could handle stereo. Neural hasn’t confirmed whether this is the case (and it’s a little bit of a niche topic), but I think we’re just stuck with mono.