I don't know your keyboard at all but I can say that in my keyboard life I have had some where the patches themselves had stereo panning built in and a summed output took away from the texture. I also don't knew gear down to model number level like some do but I will guess your mixer is at least 8 channels, right?

Which brings me to the next obvious question... don't you have another cord so you can plug the keyboard into 2 channels of the mixer and run it stereo? If it is truly the case where the keyboard has sounds designed to be played in stereo for optimal sound, roll with it.

I recall a time where I was using 3 boards and 3 modules in my rig and had a 16 channel mixer so I could run those 12 outs in stereo. Our PA mixed out to the house in stereo and some of the digital piano and string sounds that were stereo sounded AWESOME as they panned back and forth from left to right stack. Of course it took 2 direct boxes, 2 mixer channels, etc.... but we had the room.