In the second diagram you multiply the two regions to calculate a full system availability. In all cases you are calculating a probability that the system shall be down (or up).
There is however, no reference to the worst case scenario where each component fails in a serial manner, and the duplicated components fail together.(i.e. traffic manager fails, comes back as both app services fail (or app-A and SQL-B), they are in turn restored as both SQL database fail)
So a dual site solution can't provide any more guarantee than a single site solution in the worst case. But can only reduce the probability of an issue occurring.
