Research has shown that twins tend to lag behind their singleton peers in language development. There are multiple reasons for this that are individual to each twin set. One possible reason for late talking in twins is the tendency of some twins to invent their own language, called "twinspeak"; another reason may be that the twins were premature, and need time to catch up. Another possibility is that parents of singletons just have more one-on-one time with their babies, which encourages earlier talking.
By learning common sign language gestures and teaching them to your twins, you can help them learn how to communicate. Studies show that babies who sign often talk sooner, too—because they see the benefit of communication (i.e., they get what they want when they want!). Also, a California study indicated that babies who sign often grow up to have higher IQ's.
My husband and I began signing with our babies at meal time. We thought this would work best because when they're in their high chairs, they are a captive audience! First, we concentrated on head shakes—yes and no. Both twins began using these signs immediately, both with us and with each other. Then we learned the signs for "milk", "eat", "all done" and "down" and use them at all meal and snack times. One of the twins is mastering the signs, and the other is definitely interested. We make sure to clap and praise the babies when they try to sign or talk, and they love the attention. Often, the babies babble as they sign, trying out the sounds that will eventually become words. It's fun to watch!
Next, we will introduce other signs for daily life, such as "bed", "play", and "bath". As the babies get better at signing, we'll see if they create their own signs. One of my friends has a son who made up many signs of his own; he enjoyed teaching his signs to family and friends.
To learn baby sign language, check out the link at the end of the article; the site includes short video demonstrations of the signs, which is immensely helpful.