This was a Google suggested search so i decided to do a quick tutorial.
First, open your stereo media file.
Insert>Media File
Once you have uploaded your file,time to duplicate.
Right Click Track>Duplicate Selected Tracks
Once duplicated,we will split original to the left.
Original Track:Right Click>Item Settings>Item channel mode: Mono (Left)
Now repeat for the duplicate track we made.
Duplicate Track:Right Click> Item Settings> Item channel mode: Mono (Right)
There you go. I'm sure there is many ways to do this but i find this quick and easy.