Do you use the removeable media wizard to tell BackupExec you are swapping drives? or do you just swap them?
You can try Pause/Unpause on the folders to force the change
(Are you rying to do this with both removable drives mapped to the same drive letter? Veritas Symantec have always said that you cannot do this, but must have a dedicated B2D folder on a dedicated drive letter for each physical disk)