I would imagine the TC vocal signal gets summed into the keyboard and the overall sound thru the keyboard L/R is stereo out both speakers.
There is no summing happening, as the mic is mono already. So is your mic input on the EA7. The disadvantage to using this harmonizer in a stereo setup is that your voices are always mono. In my PA900, (and others) the harmony voices can be split left and right. Even though I often use a mono house PA, I monitor in stereo with the on board speakers, and it's a full, rich, vocal spread. You are getting the same signal through both sides of your system ... not a bad thing, but stereo vocals shine even better than stereo EP samples.