If they cloth diaper, diapers are definitely a great gift, but it adds up quickly. $50 will get you a couple of covers and small a pack of prefolds, or two-three AIO diapers.
I can't remember the price, but I got my cousin's wife a breast friend nursing pillow for twins. She loved it and used it forever. But that depends if she plans on nursing. Cousin's wife EBF and she felt it was easier to get them both fed at the same time.