Strictly speaking, the logical behaviour is for multiple unavailability checks to evaluate to true if the intersection of their requirements applies. For example, #unavailable(iOS 13, watchOS 3, *), #unavailable(iOS 12, tvOS 12, *) should evaluate to true on iOS 12 and earlier, only.
This isn’t an important use case to support in today’s Swift, but it might come up in practice if we had macros or availability aliases.