I would start with a place like sittercity.com or nanny4hire. I am currently looking for a p/t nanny and have an ad on sittercity and have gotten lots of responses. Also, have you tried finding a Mom's of multiples group. Some of the women that belong might be a good place to find recommendations. good luck with your search and congrats on your pregnancy twins are so much fun.
Re: Live-in Nanny for Twins in Center City