I have the setting “Backup data to iCloud” set in both my iPhone and iPad DTTG.
You telling your iPad and iPhone to backup to iCloud (in the devices’ Settings) app does just that-- backs up the DEVICE to Apple’s iCloud servers. Whatever is on the device (iPad and/or iPhone), including DEVONthink ToGo application and data (as it exists on device), goes there at intervals controlled by Apple.
My four databases total 14.8GB on my Macs, but my iCloud data shows 24.7GB used by DTTG.
That is irrelevant comparison. on your iPad and iPhone devices there are more than databases. Also there is the DEVONthink ToGo Application. Apple’s IOS consolidates all an app’s data inside the app, somehow or another.
All syncing is done by Bonjour no iCloud or Dropbox, though I have used these in the past.
Since you say all syncing is done by using the Bonjour protocol (and not via iCloud), then it is just that. You are not using the iCloud Servers to do any synching. If you told DEVONthink ToGo and desktop DEVONthink to sync via iCloud then your computer and devices would use disk space on the iCloud server independent and in addition to any device backups disk space to do the syncing.
When I stopped using iCloud I cleaned the database, so I don’t think the extra iCloud usage is a left over.
“Cleaning” is the word used in DEVONthink to “clean up” the database, which includes deleting it I guess. Bottom line is that the database disappears on the device after cleaning.
Am I correct that iCloud syncing and DTTG backup to iCloud are completely separate?
So yes, the DEVONthink ToGo iCloud synching (that you are not doing) are indeed completely separate from your devices backing up to Apple’s iCloud.
Is this as simple as the fact that both my iPad and iPhone are backing up separately to iCloud? If so I should just turn off one them.
And you can turn off what backups you want, but if you turn off your backups on one or the other devices, then that device will not be backed up. That’s normally not recommended, but up to you. I certainly would not do it.
In wrap-up, please remember that synching is NOT a backup of DEVONthink data.