unrealBook version 4.44 can sync updates to your files across iPads signed in with the same iCloud account. If you turn this feature on (settings>global>functions>Sync new files with iCloud) then any updates you make to your files will sync across iPads via iCloud.
There is another toggle called Allow iCloud to delete local files. If this is on and you delete a file from iCloud (by swiping left in the All Files list or by other means that delete files), then all iPads will have that file deleted.
This iCloud document sync once turned on, will update across iPads as you edit or add files. However the iCloud folder will only contain the new edits/files. It is not a complete backup of all of your files. If you want to have all iPads mirrored exactly, there are 2 ways.
1: Make sure all the iPads have the same files by using earlier manual methods (iMazing, iExplorer, Finder). Then turn Sync new files with iCloud on and from then on, all iPads will reflect the new files and changes.
2: Use the Settings>Utilities>Sync all files to iCloud. This will send all files to iCloud and will attempt to sync everything. This could take a long time depending on the number of files. But in the end, the iPads will contain all the latest files.
For space and speed reasons, #1 is recommended.