Splitting an iTunes Library
I have all my movies on an external hard-drive as well. There's an easy fix:

Delete all movies from the library, but keep the option for automatic organization checked, so iTunes manages your music etc.

Now drag all your movies from the second drive to the iTunes sidebar over the Movies folder. Press and hold the 'alt'-key and release the mouse. This created links to all your movies. You need to organize the Movies-folder by hand, but that's not that bad.

See also this question: iTunes videos on a different harddrive?