Two copies solves more problems than it wastes space.
What you're describing is concurrent access to a shared resource, which means we now need to start having the following discussions:
- Who's accessing?
- Where's the thing they're accessing?
- When are they starting?
- When are they done?
- What happens if they stop talking without saying goodbye?
- What happens if changes from User A happen before User B gets a full copy of the starting work object?
- What happens if User B deletes everything? Should she be able to?
- How do we manage identity?
- What happens if the work object is asked to save but is in an incomplete or non-logical state?
- What happens if the work object is damaged?
(Now multiply the word 'user' by multiple axes: actual humans, programs, network services, filesystem services)
This is an awful, awful, awful lot of work designed to save the space used by a duplicate 300KB .PPTX file. And frankly, I've only ever seen SubEthaEdit get it right.
Also: the user will never, ever, ever, ever decide to review or delete an old file. That's shit-work, and we are mortal.
A majority of customers aren't even aware that there can be multiple versions of the same work or that the work they're doing can be expressed discretely - they're just doing stuff, and cannot be expected to think about about stuff--.
So in short, let's waste some space and save some hassle. That's what flash chips are for.