This isn't two things, but if she is planning to breastfeed a twin nursing pillow is essential (at least it was to me :)). They're a lot bigger and shaped differently that the standard boppy. I don't thing I could have managed to breastfeed without mine.
That said, boppies are great for tummy time, so it certainly wouldn't hurt to get two.
I TOTALLY agree with the suggestion of the travel swings - yes yes yes- those were all my girls would nap in for a while - those things saved mine and my husband's sanity.
We were actually able to get by on only one crib. My girls were teeny, though, and liked sleeping very close to each other. However, I'm betting they could get by on one crib for a while. Bassinets were kind of useless for us - the travel swings were more portable and the girls were much happier in them.
What else? Umm...carseats, of course. (Like y'all hadn't already thought of that!:P)
Highchairs, definitely. My experience was that muliples do NOT like to take turns eating...heh. This also ties into the boppy/travel swing thing - I know plenty of twin and triplet moms who were able to bottle feed more than one kid at once using those things!
A lot of this stuff will be easier to figure out after the boys are born...being identical, they are likely to have similar tastes (as in types of pacifiers or bottle nipple or whatever) and similar needs (for example, if one is prone to reflux, his brother will likely be too...so mom and dad will require a lot of burp cloths). So telling everyone to give twice as many Target and Babies R Us gift cards would be a good idea ;).
Also - in my experience, simpler is better with twins. Cute, comfy sleepers with feet, or little snap front rompers got used a lot more than the elaborate outfits with coordinated socks. I had so much bottle and breast pump equipment that I just bought a large clear rubbermaid container and kept it all on the kitchen counter. I used those big white cloth diapers as burp cloths (those really are the best burp cloths ever IMO)...most of my girls diaper changes were done on a mat or towel on the floor. I didn't use the changing table much at all, alhough it was great for storage.
And GATES. They'll need lots and lots of baby gates, and every type of baby proofing device in the world. I'm firmly convinced that twins do everything sooner and get in more trouble than singletons because they are able to egg each other on.
HTH - and congrats to your friends :)! They are in for a wild but wonderful ride.