The problem you're having right is you have set this up to make one filesystem look like some separate ones.

But that of course means the system then thinks they really are different i.e. it will run separate file system instances then won't it? I assume that's the basically the answer to your "why not"?

Can't have the best of both worlds maybe?

It must be somewhere in between (one fs and two) though since really it's a single file system that needs to be kept in sync.

Sorry don't have an answer for you.