I'm not sure what you're doing. These are just some thoughts on the matter:
You're feeding at least 2 audio signals from MC into a mixer. My natural thought is that the mixer is going to be used to mix the sounds you are feeding it. But it only has a single USB interface that supports two channels, so you can't do any mixing in the mixer. You'll have to do any mixing of MC's outputs in software with MC's volume controls for each zone. You can only control the overall level of the USB feed. At that point, I guess it's kinda like a sound card with one stereo output. Maybe you're mixing other things with the mixer at the same time that I don't know about.
Generally speaking, hardware volume controls and hardware *mixing* controls are easier, faster, more precise, and just plain better as compared to moving software sliders. If you had a sound card in your computer, you could feed analog audio from that sound card into an analog input on your mixer and have the ability to *mix* with the mixer.
Again, I don't know what you're doing with this setup. That said, the USB inputs in mixers like this one are very limited. You generally can't route the USB inputs through effects, or tone controls, or into Aux sends. In short, they don't act like a regular analog channel. They are more like an extra "volume only" input. To me, they're pretty much useless on a mixer.
If I actually wanted to mix multiple feeds from MC with a hardware mixer, I'd get a decent 4, 5, or 8 channel USB DAC (external sound card) and route it's analog outputs into the mixer's analog inputs. The multi-channel DAC is better than several 2 channel DACs because a single DAC will keep all of it's channels in sync, whereas several external DACs will drift relative to one-another.
I have a funny feeling very little of this applies to your situation. On the off chance this might help, I decided to share my thoughts.
Good luck.
Brian.