It almost seems like you are trying to create a RAID
(Redundant Array of Independent Disks) - not my cup of tea - but it might be what you are trying to achieve?
I prefer a daily (or weekly) back up - with a master and 2 or 3 slaves (destination disks) and in my case (my music Library) is synced / backed up 4 times a week to separate different - 1) master SSD, 1 Time Capsule Drive, 1 Monterey SSD drive and 1 Mojave HDD Drive - I have them set up with an email that confirms they complete - or - "incomplete email notice" if the drive is not connected - a great reminder to connect the drive if not connected -
As soon as it detects the drive it begins the back up
Sounds like you are setting up 2 folder to folder - tasks - 1 master and 2 external drives to have the identical files as the master - not sure why they have to be done at the same time - in fact I prefere to have different drives Sync on different days in case I make a mistake and over write some files ( can be located on the drive that was not updated)
Am assuming you are Using CCC6 / Monterey OS - and the Task Schedule Settings? - I have never had CCC refuse to complete the scheduled task because there were not enough changes - I probably don't understand your settings and configuration
FWIW here is a screen shot of my Monterey Clone settings
View attachment 1935512
FWIW - here is a link to my
Mojave CCC 5 configuration