Here’s a little hack if you have a stereo file and you want to split the left and right into separate mono channels e.g. if you have an interview with the two speakers recorded on L and R channels. Using the same method above, copy the stereo file into another track and click-hold the input format button to select Left on one and Right on the other. Then make a small edit (split the file and move it a tiny bit at the beginning or end) and join (command-J) which will create a new mono file with just the left and right in their respective tracks. It is mind-numbingly stupid that 8 years after this discussion Apple still hasn’t fixed this so that you can easily import stereo files into two separate mono tracks, but here we are…
Source: (3) Easiest way to split stereo tracks to two mono tracks SOLVED — Logic Pro Help
I need to remember this for mono splitting