You'll notice it is a pretty big divide between vaults.
You can have a vault view for both vaults on your client machine, so you could drag/drop files from one to another. However as far as the vault with the new file is concerned, it is a new file with no history.
I know people that work this way. Many find it difficult to keep track of which vault they are working with and sometimes build assemblies containing files from both vaults. While this is certainly technically possible, in real life, it is pretty messy.
If you intend to work together on projects, you'll want to have one vault. Though as a warning, combining vaults can be rather tricky. If you want to keep the history, I've found no other way than to do it with programming.