Caring for one child is hard enough. Handling two -- or more -- can be total madness if you don't correctly prepare your sitter.
The first time you leave your twins or multiples with a new sitter, give her a detailed list that explains the following:
Go over your notes when the sitter arrives. Make sure you're on the same page before you leave.
If you're going to be gone for a very long time, or if you have more than three children, you might want to hire more than one sitter. If you know and trust your sitter, you can ask her to recommend a friend, but make sure you trust her friend, as well. Give both sitters clear guidelines, letting them know that you expect them to focus on your children, and not each other.
Managing multiples is a challenge. Prepare your sitter beforehand. If she understands each child's needs and personality, it will make her job that much easier.