Hi Marco,
Yes, if certain conditions are met, I think it should be possible.
Most importantly you need to make sure the iTSec plugin is configured to use the WP Cron scheduler on all sites. By default the iTSec plugin is configured to use the WP Cron scheduler. However if WP Cron is disabled for the site (in wp-config.php file by setting DISABLE_WP_CRON constant to true), the iTSec plugin will use the Page Load scheduler.
You can verify which scheduler is being used by the iTSec plugin on the Debug page. Read some of my posts on this forum for instructions on how to enable the iTSec plugin Debug page.
To sync the database backup creation day across all sites, you’ll probably need to disable/enable the Schedule Database Backups setting (and then save settings) on each site on the same day.
This should reschedule the database backup job in WP Cron.
I’ll try and sync 2 test sites myself, just to be sure. I’ll let you know the result.
+++++ To prevent any confusion, I’m not iThemes +++++