I have two double storey restaurants in my game and they both work well enough, although serving food upstairs does take longer. It won't help to make a second kitchen upstairs - your waiters won't necessarily take the order to the chef station on the same floor. The game just isn't that logical. Incidentally, I have noticed that, whenever my sims go to eat at one of these restaurants, they are inevitably shown to one of the upstairs tables. For some reason, the game seems to favor upstairs dining, rather than seating customers where they can most easily be served.