iTunes doesn't have any options for storing some content and some in another. Ideally you want the entire library (library files and media) all in the standard layout on a single drive big enough to hold it all, as this makes moving it to a new drive/computer easy. Once the library is split up moving tends to break things.
That said, if you really want to move a portion of your media to a new location, you can choose a new designated media folder, say no to the prompt that asks you to consolidate, then select the media you want to move, right-click on it, click Consolidate files, reset to the original media folder (again without consolidating) and finally delete the source files. Or use my script ConsolidateByMoving which moves the files and then relinks them to iTunes. If you're going to do this I would suggest you move the movies as this is normally a larger chunk of data with less turnover to manage. Also backup first!