I am not sure if an extra sync point could help. Problem is that both need to use the same database that manages the synchronization. Not sure if that can work without conflicts, e.g. when both users are logged in.
The virtual file system could be an interesting option for you, it shows the whole structure but downloads files only on demand.
Using a windows service that does the sync would be another option. It is not supported by the software itself, but you can workaround that:
Nextcloud-Client as Service? (Windows) and also this blog post: ownCloud as Service « Notes from a Sysadmin