When we went crib shopping a couple of weeks ago, we were pleased to discover we got a 10% "twin discount" at several stores. Of course, that still means we are spending 180% what parents of a singleton would spend, but hey, they only get one baby out of the deal.
While perusing the Multiples...and More blog, I saw a link to a shoe site called Preschoolians that offers a 40% discount to twins. Wow! I'll have to remember that for when they, you know, need shoes.
Useful information at twinpanic.com...I'll try not to make a habit of it.