I think that all you can do is use the best quality servos, leads, and servo arms and connectors and try to reduce the odds of a catastrophic failure. If you have one just hope that it is not on the "next of kin" control surface the elevator.
In this case, as most have mentioned here, the source of failure and its effect can be anything. By putting 2 servos on the same linkage may not buy you much because the linkage could fail (sliding mechanism, pushrod, control horn, clevices, etc..). Also, you are introducing more parts that could fail, the slider mechanism and the second servo. Having 2 separate elevator halves with individual servos might save you if the servo or linkage fails in a non-hardover condition. The servo going hardover to one extreme is only one of the many failure modes, so you have statistics on your side.
But in the end, because these are toys and we don't want to get into super complex and heavy failure mode mitigation, I have to agree with JJP and say do the best possible selection of hardware: good servos, good pushrods, clevices and control horns. And check them everytime you go flying. My two worst crashes I ever had were not because of servo failures, but because of stupid distractions causing me not to check that the servos were reversed!!!