If I know the mother wants to nurse, and it's her first baby, I usually get some kind of nursing accoutrements or a book (and "coupons" for middle-of-the-night calls for help with nursing issues, but that won't be a possibility for you). But I would not do that if I weren't absolutely sure the mother wanted to nurse because it would come off as pushy or preachy. And now that I think of it, getting a gift like that from single dude would probably seem weird. So, scratch that.
Rajah's idea is great- first find out if they're registered, which will make your job way easier, and barring that, basic baby supplies are ALWAYS needed and duplication is not a problem since they'll go through lots of them. If your budget is such that this is possible, you could go for my backup gift: a wrap-style baby carrier. They are more versatile and portable than any other kind I've found, including the slings everybody raves about (including me, before I found out about wraps). And IMO it'd be useful even with twins- there will be times when one of the parents will have both of the kids, and will need to carry one and have hands free.
The one I have is a Didymos my sister-in-law handed down to me, but they're about $150 (imported from Germany or something) so here's a more reasonable domestic version:
Moby Wrap, Inc. - Moby Wrap
Edit: Found directions on the site for tying the wrap with twins: Twin Hug Hold Wraping Instruction