You might be able to create a “C:\All-My-Backup-Sources” folder then put symlinks / hardlinks in them to the remote machines…
But there are all sorts of things to be wary of with this sort of setup. For example, if the backup runs on your machine but your wife’s machine is offline then Duplicati will consider all of her stuff to have been “deleted” and flag it as such in the backups.
Depending on your retention settings this could cause an unexpected removal of a file from the backup along with re-backup of the file when the link comes back.
Is there a particular reason you’re wanting to run Duplicati in a single place rather than at each of the potential backup source machines?