As far as I know, there's no built-in support for what you're trying to do. But you can probably do it anyway.
Use a subclass of NSScrollView and override the 'tile' method. This method is what lays out the NSScrollView's elements.
You should call super's implementation, then move the bottom NSScroller to the top (using something like setFrameBounds). Make sure you move the contentView (documentView? - can't remember which - it's an NSClipView) down so it's not obscured by the NSScroller you just moved.
I've never tried this, so I could be just talking out of my ass
