As i know, you have 2 options to copy data from DB2 to DB 1
before the action, take the backup of A.folder from DB1 (if in case it is required in future), then delete the A.folder from DB1
Option 1: select the A.folder from DB2 drag and drop this folder on DB1 (drop it onto the name of the DB1), then it will show you the doalog window for Merge, select appropriate options with target path and finish the action.
Option 2: export the A.folder from DB2 (right click on A.folder -> export -> as XML) then import it back into DB1 under target folder.
See it might be the cumbersome task, so instead i would prefer to again delete that particualar attribute value for all models present in A.folder in DB1 by writing a somple script :)