Sorry, I first checked your question with a SL2442AI, but this evening I had a look at my RM16AI. Here is what I see (and hear through RM's headphone out):
When I link aux 1+2 I see in cue source "mix 1/2" - and that is a stereo mix. But I need to check the pans of the input channels and adjust them. By whatever reason linked input channels in aux mixes are defaulted to mono. Linked channels are correctly panned in main mix, but not in linked aux mixes. "Cue" works exactly like the "solo" function. But also here: No stereo unless you broaden the pan level. I am using Firmware 1.0.7884 on RM and I remember that the early firmwares had some problems with stereo functionality.
I have no CS18AI, but as far as I know its headphone out is a direct "copy" of RM's headphone out, so it should work.
DON'T ENABLE SOLO PFL IN SYSTEMS SETTINGS! Solo PFL is mono only...
And remember that "cue" and "solo" are situated before Graphic EQ...