I have an org-mode file separated into multiple categories. I know I can create an agenda command that will show only items from one given category, like so:

("a" "Category A items" agenda ""
      ((org-agenda-category-filter-preset
        (quote
         ("+a")))))

and then only items from category A will show up in the custom agenda view.

How can I configure a custom view so it will show items from two specified categories (e.g. "A" and "B")?