The best way would to create a smartlist and have it only show duplicates of Name, Artist and Album.
Create a custom field (locations) that you can enter both, DT or LT in. All of the files listed in the smartlist should be in both so label them accordingly.
Now create another smart list and tell it to show files that the locations tag is not Both sort by location label all according to their location.
After that is done you can use the library tools Rename, Move and Copy Files feature copy all of the DT files to the LT and find replace DT with Both
repeat for files tagged LT.
This will require a consistent naming scheme to work correctly but it will work.
Another option which should be easier would be use a sync program for the files and configure MC auto import to monitor the folders.
See this backup discussion for some details on software that may do you file sync for you.
http://yabb.jriver.com/interact/index.php?topic=72117.0