Hi Patrick,
I think you can do something like this. If I remember, you can create a composite element, then move the 'child' diagram elsewhere in the model. There was once a thread with a procedure for doing this without getting into trouble. I think some posts in the thread dealt with possible pitfalls.
You will have to search the forum for this information. I don't remember when I saw it, except that it was quite a while ago, so go back a few years.
Meanwhile, I do not believe that a diagram can be a child of two parent elements. My assumption - and it is only that - is based on my understanding of how this is handled in the EA schema.
What you can do though is create a hyperlink to a diagram. This would allow you to create a glyph (or something) on the diagram with the 'foreign' activity, which would take you to the child diagram. You can also make hyperlinks transparent, and remove their text. This allows you to position them over an element, turning the element into a sort of navigation button.
There are some risks to the above process. Transparent hyperlinks can get lost when you manipulate them. If they have no text they become completely invisible when they are not selected; in ill-timed click elsewhere on the diagram can do this. Last time I looked (at least a year ago) they did not show up on the Project Browser, so they became very difficult to find if you lost them.
Finally, if you need to change the type of child diagram that a composite element provides you, no problem. Navigate to the child diagram, then from the main menu - this option is not available on context menus - choose Diagram | Change Type.
HTH, David