Note: For this to work you need to have google authenticator working on a rooted device.
Google authenticator stores user data in a sqlite database, so we can just get the key from there and move it on a different device, the process is as follows:
$ adb pull /data/data/com.google.android.apps.authenticator/databases/databases $ sqlite3 ./databases sqlite> select * from accounts; email@example.com|key|0|0 sqlite> .exit
The third column contains the key you need to manually copy to the new phone. After that, google authenticator will work on both devices (you can check the time-based generated key is the same on both).