iTunes Utility - Merging Libraries
Try this,

http://www.mediarover.com/

Haven't tried it myself yet but looks like it will give you exactly what you need.

EDIT:

Script Method: http://guides.macrumors.com/Keeping_2_iTunes_Libraries_in_Sync

I actually use this technique myself.

Creates a "carbon copy" of a library on another machine which you can then sync to reflect changes.

Works best with two Macs, never tried it with a PC.