SONGS

You just need to create multitrack song for each stereo track.
Just start to create a new multitrack song put your first song to the CH 1 and 2
Set the mixer save the song
Start to create a new multitrack song put your second stereo file to the CH1 and 2
Set the mixer save the song
Start to create a new multitrack song put your third stereo file to the CH1 and 2
Set the mixer save the song
and so on.

Now you have a bunch of multitrack song where each song contains only one stereo file.
Create a new playlist with those multitrack songs

Done