congratulations on finding out you are having twins!!
i would be doing some research and trying to speak to a few different OB's and go with one that is willing to support in a safe way what you want not.
i have couple of friends who have delivered twins naturally (one set at 36wks and one at 37wks) neither had previous c/s so i know that is different but depending on your reasons for your prior c/s will also come into affect