What about Dorothy and the yellow brick road?
Jack and the beanstock (green sweats with fake leaves) ; Dish and the spoon (Hey diddle diddle)-or a cat and fiddle (a violin on a onesie for baby) for that matter; a big and small version of pretty much any animal, or something like a cat and mouse; Hansel and Grettle (sp?); Cinderella and pumpkin or a mouse; Ariel and sebastian or flounder; Pooh and piglet or a pot of honey; Eeyore and his tail-ok that might be a little weird
but maybe the other could go as a pile of sticks aka eeyore's house ; or you could always go with a theme like sweet and sour with one as a dessert of some sort and the other as a lemon or something (ooh I need to figure out how to make a sour patch kid costume, hmmm...)
I have always wanted to do a theme using the colors of one of my favorite teams, for example gold could be represented really easily by drawing the Au symbol from the periodic table on a t and blue could be blue (the dog). Together you have gold and blue (go 'eers). this way you could relate the two and easily make costumes appropriate for each child because there are so many options to represent most colors. I guess this only works if you are a sports fan though. (I hope that made at least a little sense
I am going to stop now, because I could go on forever if I let myself.