use “select scenario disks” and then “insert scenarios”. It’ll make those disks the default and insert them in the drives. Do that whenever you want to switch default scenario disks.
Are you running 2 instances at the same time? That wouldn’t work.
Here’s another thing that should always work (as long as you’re running a single instance of the game at once):
Back up your scenario files in the saved games folder to another folder, then move the ones you want into the saved games folder every time you want to run a different party. Keep your different scenarios in their own folders and move them to the saved games folder before playing.