I think just a simple congratulations card would work!
.. is the baby shower a joint shower for both of her babies?
These are some ideas (sorry, they're all from my gallery... just ones I remembered making... I'm sure there are others out there as well!)... if you wanted to do them more unisex, I put some ideas....
Gallery at Splitcoaststampers
On this one, maybe you could do one foot in blue and one in moss? Just to sort of show the "2 babies" thing?
Gallery at Splitcoaststampers
This one you could use the words "always" or something on it instead of grateful.... and obviously different colors....
Gallery at Splitcoaststampers
This one can really be unisex depending on the colors.... and if you put "love" iinstead of thankyou and even wrote "babies" instead of baby....
I hope this helps give you some ideas!
Kimberly