why not just use the categories and set two top level categories, pewter and silver, then have them filled with subcategories.
Then its just a matter of setting the category llist to show subcategories all the time.
Usually the subcategories are hidden but you can change that.
That would give you what you want, two sets of categories under two different headings and all in the categories sidebox, not split into two and using the documents box as a category.
