The most helpful game I've seen is a "drawing" (though it doesn't have anything to do with twins):
have a really nice plant/flower arrangement as a centerpiece.
get "thank you" notes that match the party theme
take the envelopes, a pen, and a large bowl, and place them in a convenient location
everyone at the party fills out a thank-you envelope addressed to themselves
at the end of the party, the mom-to-be pulls an envelope out of the bowl and THAT person gets to keep the centerpiece
the mom-to-be is gifted with the envelopes and thank-you cards, so that she has an easier time getting them out
Have fun!