Adele Brodkin: I am glad to hear that the twins do talk at home, and hopefully, their language development is close to age appropriate. It is very common for twins to have some delay in gaining age appropriate facility with their native language, and even to have a special substitute language which is used only with each other in these early years. Even their parents may not understand what they are saying to each other. Some experts suggest that twin-ship slows down the need to communicate other than with each other.
I would suggest that you try "tricking them into talking". Here's an example of what I mean: assuming they know their colors, hold up a crayon or a lollipop of a given color for one twin at a time, in private. Make it clear that naming the color will be rewarded by your handing over the desired object; or you might do something comparable with the naming of a desired toy or game. You would know whether it is food or toys or privileges that are most tempting to these two. There are various games you might think of that similarly reward speaking.
Also, try to encourage each to play with kids other than each other or at least in addition to each other. We sometimes see one twin who is the spokesperson for both. If you occasionally guide them toward different "teams" or different activities, allowing one twin to take over becomes less of an option.
Be patient, but if months go by without some progress toward speaking even single words in school, gently suggest to the parents that they consider a referral to a speech therapist.
For more advice by Adele, check out the Between Teacher and Parent column.