You can…but recommend testing it first. When you look at the code for it, you will see where the first thumbnail is. For example, on the demo site, you see one with the single thumbnail source:
All you would do is add another below or wherever you want. Please note though if you need something specific, this is where you may need to do some custom html coding. Technically this boxed thumbnail feature was really made for a single image or other media type.