It depends on the total calculated load of both dwellings. Your electrician should be able to figure this for you. For example: If the existing dwelling is 2000 sq.ft. and total electric I would guess a sub feed to another dwelling of any size would not be a good idea. You simply need to calculate the total load of both dwellings to determine if 200A service is adequate. The only reason I can think of that would definitely prohibit a second service is if the dwellings were to be connected and not have proper fire separation.
