1. Click the gear icon (upper right in Universal Control)
2. Choose Digital Patching
3. Choose Input Patch
4. Choose Analog Send
5. Scroll all the way to the right -- you'll see Main L and Main R near the right, next to Solo L and Solo R
6. You'll see Mix 1 and Mix 2 on the left near the top (scroll if you don't).
7. Click in the boxes to route Main L to Mix 1, and Main R to Mix 2
8. NOTE that whatever you're using as an amplifier may either require +4 or -10 levels. If you get distortion on the output and your Main L/R isn't overloading, try backing either the main fader or your mix master faders for 1/2 down to -12 and see if the distortion goes away. I ran into this.