UPDATE: The tool supports re-running a migration and only pulling across changes. It is still only one-way and is designed to support migration.
https://github.com/nkdAgility/azure-devops-migration-tools
The intended purpose of this feature is to allow you to migrate many work items over a few weeks while users are still able to access the source. You can then re-run the migration with a special flag and have it pull the small number of changes.
WARNING: For sync you should note that the Source always wins. So if both the target and the source have been updated then the Source will overwrite the source.