As for the CM-300, I don't know. Could you be plugging the cables in incorrectly (i.e. left from one into the right of the plug at the other end)?
As stated in my last post, I've tried two different cables and two different inputs - result is the same
Otherwise, maybe it is all backwards. But if it genuinely is two synths in one box, then it might be on your end.
That's the CM-500 - CM-300 is just the SC-55 source, the reason I was asking if the SC-55 was reversed too
My guess is Roland reversed the CM-300 to keep compatability with the rest of the CM series.