When having a birthday party for twins, it is really important to have different cakes for each child.
Children that are twins have to share so much that it is nice if they can have their own special day to reflect some of their personality. Keeping everything the same seems cute, and is okay for toddlers, but not so for school aged children.
Even if the décor is the same, you could offer two different parties, or if cost is an issue, you can simply ask each child what type of cake they want.
At least by offering two separate cakes at the party, each child can express his or her individuality without breaking the bank. Asking the children for input is a great way to plan the party.