Editing the server URL is not really possible, because internally this is a very complex matter and there are many possibilities for data loss because data has internally to be copied partially (only the unsynced data?) to a new calendar/address book. There are so many cases where this could result in chaos, so we probably will never add this feature. All you can do is create the second account, export unsynced data manually on the device from the first account and import it to the newly created second account. There are helper apps like ical Import/Export afaik.
