Login as the user with the smaller of the two iTunes libraries. You'll need to back this library up, so picking the smaller library will save you time.
How to: merging iTunes libraries between users
by CNET News.com | November 5, 2008, 1:06pm PST | Image 1 of 4
We're going to show you how multiple users on a shared Windows computer can merge their iTunes libraries. This tutorial assumes that the users on your computer are set up for administrative access, which allows you to see and modify each others files.