This applies to both v6 and v7, but afaik you can only check it natively (as opposed to using a plugin) using the v7 double-sided material:
- create a double-sided material
- click the link to assign its front material
- choose plastic and click apply, the plastic appears as child
- create another double-sided material
- click the link to assign its front material
- switch to the “start from existing” tab in the content chooser
At this point you can only see the plastic material from step 3 if you enable “show tree view” and expand the double-sided material – it does not appear in the listview, and if you select it in the tree view and click apply, it will be the parent double-sided material that is copied or instanced.
In v6, the message at lower-right in the content chooser will read “2 contents selected” when you have selected the child plastic material in the treeview.
Perhaps curiously, if you perform this test using the front & back materials of one double-sided material, instead of using two different ones, the material will actually end up getting parented to itself, not sure if that is expected.
The only apparent workaround is to create the child content on its own first, so it appears at the top level, before then proceeding to use the “start from existing” tab to copy or instance it under other contents.