Yes - Mixup can link directly to the 1/4" outputs... but you lose two key features of using the Stereo Out 1U module:
1. The output from mixup has no master volume - so whatever you have plugged into Input 4 L+R will be output at full volume to whatever you plug into the 1/4" outputs.
2. The output from the1/4" jacks will be unbalanced. I noticed a significant reduction in noise when I switched to using balanced I/O between the modular and my UAD Apollo 8 interface, and having that master volume knob is awesome for taming the output.