You can do this, you define the databases under Multiple Databases Definitions from the main page of Solo, not the Configuration page. The instructions are here: https://www.akeebabackup.com/support.html?view=ticket&id=24952#p137917
Just because you can doesn't mean you should. You are right, the backup is easy. But to restore a single site from this type of backup is a nightmare. When you restore this multi-site backup, it will restore all of the sites. That is not usually what you want to do.
You can easily set up a backup for each separate site using different Profiles in your Solo installation. If you set them up as CRON jobs, they go off in the middle of the night and you don't have to watch all those separate backups happen. The extra effort will pay off when you need to do a restore.
Dale L. Brackin
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