Premiere Pro does not allow you to convert mono to stereo or stereo to mono. The only way to keep your left and right channel on their own respective tracks is to make sure your sequence settings has only mono tracks is to create a new sequence and set your master audio to the appropriate settings, enter 0 for stereo tracks and the number you need for mono tracks. If you are starting a new project just do what was outlined above only you will not have to create a new sequence in this case.
You may also want to create a custom preset with settings you will use commonly. This way you won’t have to manually change settings each time.
Hope this helps.
Also new to Premiere Pro.