What a cute idea! I love that! I also have a cousin expecting twins, and although I am not really helping to plan the shower I am helping on the day itself. I am going to steal your idea if you don't mind and pass it on the cousin who is planning it. :D
I will think more about it, but I did come up with Twix candy.
You could also carry on that theme by doing "double" food. Like double chocolate cake (or marble if the babies are one of each gender) double decker sandwiches. I'm sure there is a lot more stuff, but I can't think of anything else right now. I can't wait to see what other answers you get. I bet you could find help with the poem for the invite if you just google "twin shower invitation". I got the poems for showers I have done that way.
Also, don't forget to think of things with TWO in the name...