How can I let two iTunes installation use same iTunes folder?
If you use one remote library location for iTunes on two machines [real or virtual] then even though the data location is shared, no other information is.  

Each iTunes believes it is fully in charge of the library.
It is unaware that any other iTunes is changing data in the library &amp; won't spot that a file already exists which it doesn't already have in its database.

In essence, iTunes is a write-only structure, it doesn't read the info from the library itself, only from its own Library.xml &amp; .itl files. Dropping a new file into the Library by any method, including from another copy of iTunes, won't make the other iTunes notice it's there.

One workaround - which I'm not sure I'd actually recommend testing without solid backups - could be to symlink the .xml &amp; .itl files together, across both machines.
There's a guide on symbolic links on HowToGeek
Essentially, linking the two sets of library files as one single structure, which either iTunes could read/write to.  

Obvious downsides would be
a) there would be no lockfile protection, were both iTunes attempting to write at the same time.
b) the file hierarchies would have to be identical on each machine, as iTunes file references are absolute, not relative.