Halloween costumes that "go" together?

2 of my 3 kids (boy and girl) are obsessed with having matching costumes on Halloween. They have been: Thing 1 & Thing 2 from Green Eggs and Ham, the Critters from the Mercer Mayer books, Arthur and DW, Harry Potter & Hermione Granger, Gandalf and the Balrog, pirates, a princess and a frog. This year they are talking about being and Egyptian Princess and a mummy.