The easiest way to accomplish this would be to throw an alert as Douglas has suggested to inform the user that they should select multiple parts.
It may also be possible using CUI which allows you to define an Init Method that can run logic to determine when a menu item should appear. We have a few blog posts regarding customizing menus using CUI but none of that currently cover this exact use case. This is an interesting requirement though, and I can certainly add this to our backlog of future blog ideas.
Aras Labs Software Engineer