I Merge to Stereo Wave using the Audio > Merge and this saves a stereo file in my work folder for the song. This wave is then imported back into the RealBand song to a MIDI track and gives the options successively to select Stereo, split channels to separate tracks and then finally Mono. If the track is already set to Stereo then this setting is used immediately by the imported file.

Check also Pref > Audio > Import Channel for Mono Tracks - this can confuse the issue. Mine is set fo Left Channel.