I don't think you would gain anything by having a separate server instance (on the same computer) for two different collections. JRiver makes it easy to have views that show you only what you want. You could very easily build a view that shows just Classical, and a view that shows everything else. You could switch views with a click of the mouse.
That being said, if you really have to have them separated, maybe what you want is two different Libraries within the same Media Center instance. Media Center will only show one Library at a time. You can experiment with it pretty easily:
1. File > Library > Library Manger ... > Add Library
2. Import some Classical files.
3. Play around.
4. Switch back to your other Library: File > Library > (Choose the Library you want; the original is Main Library)
5. Switch to the Classical library: File > Library > (Choose the name you gave your Classical library)
Finally, if you wanted two totally separate Media Center instances, running *at the same time*, with different Library Media, I'm not sure how to do that on a single computer without using virtual machines.
Good luck to you.
Brian.