Is there a way to do sub-projects?  I have a couple of related projects that are part of a larger effort.  Right now I have them setup in one file so I can relate them, but I'd like to define them as separate sub-projects within the larger whole.  Particularly, I'd like each sub-project to have it's own critical path timeline.  I can do this if I copy them into separate files - but then I can't relate them to each other. 

Any thoughts?