multi-pass rendering is when you render only certain elements at a time usually foreground to background (or vice versa) then composite in a graphics program such as Photoshop. As for your other quetion, not sure what you're trying to accomplish but as I read it, you'd probably have a seam at the horizon line regardless of how you proceed. Have you considered using prop like Daz' cyclorama, RDNA's infinity cove? If your background image incorporates the gound, that could do the trick allowing use of shadows in a somewhat "realistic" manner.