All you have to do is press the I/O key, tap Outputs, then Mix Out (both on the left side of the screen). Make sure you’re on the Local tab at the top as well; you probably are, though.
Then, scroll down to find the MainLR L and R rows. Scroll right to reveal the A and B columns, then tap the grey square at the junction of MainLR L and A, and Main LR R and B, so they go white. You can have the same mix going to multiple outputs, so this won’t affect any existing patch.
That’s it – you’re all set.
This is pretty basic I/O routing stuff, you should definitely check out the reference manual starting on page 17.