I have twin boys and my shower was beautiful and special without the twin theme. You can really go overboard with the whole "twin" thing and I really wasn't into it. I wanted my kids to be more individuals and I almost never refer to them in RL as twins.
Another friend had a twin shower with the 'pea in a pod' theme and the favor was salt N pepper shakers. They were cute, but how practical were they for the guests? You could do twin cookies or something edible. I'm sure if you google you'll get lots of ideas.
I don't know of any twin games, just the regular old shower games, I personally like bingo to keep people quiet during gift opening.
I could not live withough 2 bouncy seats, 2 swings, and 2 high chairs. Most other things you really only need one of, like a pack-n-play with a changing table attached. I loved my baby sling, you can easily carry multiples with 2 slings or something like a moby wrap. I am pregnant with my 3rd now and already have purchased both as much haves for this baby.
The best thing you can do for her is prepare food and offer to help around the house after the babies are born - not to help with the babies, she'll want to do that, but maybe prepare a meal, clean, or do laundry.