What makes you think it would make sense for two small startups to just "join", especially in this space, where it's not like you can just plug the technology of one into the other.
What makes you think they can't? They have the same target audience, some of the same investors, and are working on the same problem - universal access to one's files.
I'd:
* Rebrand Zumodrive as a future version of Dropbox
* Use Dropbox's syncing model to ensure the local cache is kept up to date to a user-configurable percentage.
This would allow speedy access where the local device has the space, and still allow access where the local device doesn't have the space.
