I’m on Mac, Cubase 10 (tried with 9.5, same issue):
I have a Movie Stereo Audio Track with dialogs on the Right Channel and Music on the Left Channel.
To hear them separately on both the Channels I used to create a Mono Group in which the Audio Track got routed.
Working with the Pan of the stereo Audio Track, I was able to select Dialogs or Music.
Actually, doing all this, panning the audio Track, I only can hear the Left Channel !!