It is indeed possible to combine two or more *.obf files!
Download Osmand's Mapcreator for Desktop Computers with Java RE ...
try to get the nightly-build of MapCreator from
osmand.net
In that zip archive you can also find a file inspector.bat or .sh
Run that file via the commandline interface of your OS with the
parameter -h to get detailed instructions about merging obf files.
By the way, there is an open ticket about the offline address search
in Osmand app after merging obf files:
http://code.google.com/p/osmand/issues/detail?id=438
Can we solve this issue?
Greetings from Stephan