The script handle.bash (downloadable here) can be used to replace all database handles in a .gramps file. This makes it possible to work with multiple databases in .gramps, merging select branches manually at will. Without this tool, manual merging of databases derived from one another or with a common base-database would not be possible because of handle collision.
In order to use this script, you must create a directory and copy the script file to it. Then you export the database you wish to import into a colliding database to a .gramps file inside this directory. Then you run
$ bash handle.bash filename.gramps
where filename.gramps is the name of the file you exported into the new directory.
The script will create the files file.gramps.zcat, handle.old, handle.new and file.gramps. The file "file.gramps" is the final output, the original "filename.gramps" with all handles replaced.
This script is released under the GNU GPLv2, author: lcc.