RE: Copy & Paste functionality?

It's not currently possible to copy & paste between diagrams in ArgoUML.

The easiest way to split your diagram is to create a new diagram and drag
the elements from the explorer pane onto it.  All the connecting
association, dependencies, etc will get redrawn automatically as you add the
elements and you'll "just" have to worry about positioning the nodes.  If
you've got a printout or another window with the existing layout you should
be able to work relatively quickly.

Add your vote to the enhancement request for this if you'd like to see its
priority increased.

Tom