to merge:
* if the 2 datafiles are in the same filegroup (PRIMARY): use this: DBCC SHRINKFILE WITH EMPTYFILE, after that, you should be able to drop the datafile from the database
* if the 2 datafiles are of different filegroups, move all the tables and indexes to the primary filegroup, then you can drop the datafile.
if you don't know how to move tables/indexes from a filegroup to another, check out google, it's easy to find.
