In theory, you should make these diagrams “Unsynchronized” (read more on this topic on this thread: How to disconnect cloned diagrams? - #7 by SamuelRochet ) - This will cancel the behavior that makes sure that your diagrams reflects model content. Nevertheless, when you do so, relationship between elements (edges) are not displayed anymore when creating new elements, which is a big drawback. I believe this is a behavior that should be changed…
Anyway, I guess the only workaround I see here is for you to work on a single diagram, and then when you’re done, clone your diagram and hide what you want in the other one…
Stephane