Mars doesn't make a ton of sense as a backup earth because gravity there is too weak to carry a fetus to term. If we had the tech and the urgency to terraform mars, it would probably make more sense to put those resources into space stations where we could control the environment better (and less risk from losing any one station). Mars in this scenario is sort of like an offshore oil rig that people work on for some months at a time and then go back to their life in space (again provided we can't live on earth for some reason).