Welcome to the forum!
The short answer is no, you cannot define complex sequences and dependencies like this. I’m curious about your use-case and why this would be important.
I understand that you’d need to know how long the backup job will take, but keep in mind that Duplicati is a type of “incremental forever” backup. The first backup will scan and process all files, but subsequent backups will only scan new/changed files. For changed files, only changed blocks within those files are backed up again. So you may find that your second backup and beyond are quite fast.
If you have concerns about when exactly files will be backed up in relation to each other, a possible mitigation would be to use filesystem snapshots (like VSS on Windows). When enabled, a snapshot is taken at the start of the backup and all files will be backed up exactly as they were at that moment in time. You don’t have to worry about files being modified during the backup window.