I'd like to have the same pages assigned to two different parent pages, to save me making separate pages for two sections that contain mostly the same info. I'm using the WenderHost Subpages widget to generate the side menus.
Not possible natively, parent is simply single field in database.
There are plugins that allow to create more complex relationships, such as Posts 2 Posts but it's unlikely to work with just any widget.
Alternatively you can create custom menu with arbitrary structure and use native widget to display it in sidebar.