A key consideration is load time. If your project is light on bitmaps, a single scene could do it.
If you use multiple scenes, you have to decide out how to animate between them and figure out where the start and end coordinates need to be or visitor might end up having to jump around with their mouse.
Here's an item where the first and third scenes are standard canned animations with the second scene having one huge parallax project. (Note the load time - this is socked with bitmaps.):