db-dump-restore.txt

A quick google for 'sqlite3 dump and restore'  returned - https://www.sqlitetutorial.net/sqlite-dump/ and https://stackoverflow.com/questions/311691/importing-a-sqlite3-dump-back-into-the-database

This worked for me in minimal testing....

# dump the two db to files
echo ".dump archive" | sqlite3 /home/pi/weewx-data/archive/db1.sdb > db1.dump
echo ".dump archive" | sqlite3 /home/pi/weewx-data/archive/db2.sdb > db2.dump

# read them both into a new file
cat db1.dump | sqlite3 newdb.sdb
cat db2.dump | grep -v CREATE | sqlite3 newdb.sdb

The differences in the read commands above are because a db dump has a command to create the archive table when it creates the new db.  You have to exclude that when you read in the dump files from the second one.

If you see errors like "Runtime error near line 3: UNIQUE constraint failed: archive.dateTime (19)" you can ignore them.  All it's saying is that record already exists in the new db, which happens if you run the commands