I am currently pregnant with fraternal twin girls and truly applaud your desire to avoid the "matchy matchy." If you know the girls' names, I would say getting two, beautiful, soft, warm baby blankets (not-identical) and having the girls' names embroidered on them would be perfect. The weather will be cool when the girls come home and the first pictures will have the girls swaddled, so having beautiful blankets will be nice.
You could make a creative log for them to use after the babies are born. You can look on-line for templates. When the girls come home, they'll need to keep track of diapers and feedings and the like. Perhaps a log that goes on the fridge and can be filled out with a dry-erase marker?
When I was pregnant with my son, one of the most touching gifts I received was a fabric book my mother made with pictures and information about the family. (She sewed the book with quilting batting to make it plush, and used print-at-home iron-on transfer paper to put the pictures and information on the inside.)
Also, perhaps the nicest thing you can do, is offer to deliver food during the postpardum time. They will have their hands full, and they will appreciate not having fast food all the time.
You are awesome for being so thoughtful for this couple. I wish I had more friends like you. :-)