In a nutshell, I'm trying to merge two .OSM files. One of them consists of only administrative boundaries and the other consists of a bunch of data (motorways) without any administrative boundaries.

I plan to merge the two files together.

Here's the osmosis command that I was using:
osmosis --read-xml largermessedbaroundaries.osm --read-xml boundariesonly.osm --merge conflictResolutionMethod=lastSource --write-xml boundariesrestored.osm

(I realize that I misspelled boundaries in one of the files' names but have decided to not rename it for consistency).