Double booking, while it may seem smart, it’s not entirely fair to people who are trying to make committments to eating at certain times at certain places.
I never knew about the “auto delete” scrubbing that they do, but that would probably explain why some people get their reservations lost occasionally.
The reason that some castmembers will allow it is because they don’t actually know all the rules. There are a LOT of them. Which is why sites like this are so good.
My best recommendation is to make the PS for the place that’s most likely to book up, so that, like Dana said, you can just show up and get a seat for the other one. You can always call on the day of the PS, or once you’ve firmed up your plans to see what the deal is.