Even the mechanism predating WALv1 (rollback journal) uses two files. I don't think there is any way of using SQLite crash-proof with just a single file.
Besides, even if the database is single-file it's still necessary to use filesystem snapshotting for live backup, or it's likely to get an inconsistent copy.
Besides, even if the database is single-file it's still necessary to use filesystem snapshotting for live backup, or it's likely to get an inconsistent copy.