Brute force approach: Send it the “.dump” command to create a text representation of the data. Read that data in into the second database. Not sure you can use that.
If you need a fine grained update (sending a copy of each upto the other copy), have a look at sqlite3_update_hook
But how do you plan to handle errors? For example, what happens when the copy of the DB in app2 can’t make an update for some reason?
To solve this, move the database to a server process and have the two apps talk to it.