Please, read the documentation before asking for help. The -add-db-pre option does exactly what you want
OK, I'm figuring this out. Every time you execute
sudo webinoly -backup=s3 -profile=<profile-name> -add-db-pre=example.com -destination=/var/www/example.com/db-backup -max=10 -bucket=<bucket-name>
an entry is added to ~/.duply/<profile-name>/pre. I now understand that I need to add one for each site whose database I want backed up. Practice makes perfect!
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