The Kontakt Player 2 can not be purchased separately; it only comes as part of an associated sample library. The Kontakt Player 2 can only load and play other Kontakt Player 2 libraries, not any Kontakt 2 library.
The easiest way to understand the Kontakt Player 2 engine is to compare it to the older Kompakt-based libraries. Under the old "Kompakt system", each library has its own separate player interface (e.g. GPO player and interface separate from EWQLSO Gold interface). With the Kontakt Player 2 engine, all Kontakt Player 2 libraries you own are displayed within a single inteface.
If you look at the screen in the background of this picture, you can see a single Kontakt Player 2 interface showing Garritan Strad, Fixed Noise OTTO, VirSyn Acoustic Legends, Sample Logic AIR, and Chris Hein Guitars (5 libraries) along the left hand side.