it sounds like the royal family. They never traveled in the same plane together. If one plane crashes the people on the other plane will survive and their will still be an heir to the throne.
Are you talking about the British Throne? The line of succession to the British Throne is about five thousand people long. Even if the entire (current) Royal Family were wiped out in one fell swoop, there would still be an heir to the throne.
A scenario, such as was depicted in the movie King Ralph (where the entire Royal Family is wiped out and they have to *search* for the next in line to the throne), is completely impossible. (Yes, I know it was a comedy and not meant to depict reality.)
The Wolf