Introduce this gradually and separately so they do not trigger each other in their own reactions. Begin with brief separations in which someone or something they like will distract them.
Let's face it. It's going to be hard. You HAVE to leave them and there is nothing else you can do to get rid of the problem. Try taking short trips and leaving the twins with a relative or a babysitter. Then graduate to longer trips away from the house. Eventually they will get used to it.
The best way to help them is to ease them into being away from you. Don't force it on them. Try leaving them with a babysitter for a couple hours. If they cry, just let them cry. At pre-school you may be able to attend with them a little.
She should try to discover what they are anxious about, and then explain there is no reason to worry. Perhaps she can send them to special camp to build their courage.
