I thing what Eric is actually asking is how do boxes move at different angles within a perspective piece (in relation to one another). In other words If you have a group of boxes done in two point perspective, it's pretty easy if they are all going back to the same vanishing points, but once the boxes get shifted around, it's a little trickier.
The way to solve this problem is to KEEP THE HORIZON LINE CONSISTENT, but move where the vanishing points are. I typically just freehand it first to get it looking sort of right, then find the points. This helps it from becoming too stiff. Here's a quick drawing to show two objects in two point perspective that are at different angles.
Hope that helps some. : )