+1 on the "don't do it" tip. Even if there's no risk of damage, that's a setup that has a high potential for confusion. But there might be a fix...namely, this: https://www.sweetwater.com/store/detail/BigKnobPass--mackie-big-knob-passive-2x2-studio-monitor-controller
This thing allows you to have two stereo inputs and two stereo outputs. The latter will be useful if you get a set of "tracking monitors" or "mix checks". But the dual input scheme fits perfectly here, AND it's more than likely that this is what Mackie intended this $70 widget for.