It certainly could be done on Dropbox, their services have been growing more close over time. Zumodrive only recently added linked-folders, which are basically

It certainly could be done on Dropbox, their services have been growing more close over time. Zumodrive only recently added linked-folders, which are basically a copy of the Dropbox model. That said, they are both YC companies, and it's good to have competition in this space.