Hi hachback, Welcome to the reaper forum!
Not sure if this is what you need but here goes.
If you have a stereo file and you want to have the left side on 1 track and the right side on another.
Copy the file so you now have 2 identical stereo files on different tracks.
Now right click on track 1 / item settings / item channel mode: mono ( left )
Then right click on track 2 / item settings / item channel mode: mono ( right )
now you have the left side of the stereo file on track 1 and the right side on track 2
Hope this is waht you meant
Cheers
Wolffman