OK. first, make sure CHs 17 and 18 are paired. (Find the SEL buttons above each channel fader and press them both at the same time. then, look at the display. answer Yes by hitting enter.)
disregard this if you already have this done.
Now, hit one of the SEL buttons again so that they light up green, then find the STEREO button up next and to the left of the display. Push it so that it lights up as well. Next, look to the right of the board and find another button that says STEREO next to the SOLO knob. Push it so that it also lights up. Move up your main stereo fader and turn up your CR knob.