Even though he's older, that would be fine.
Remember, they're both in good health. It would certainly make Tradition that much more secure.
BOTH of them aren't going to get hit by a car, or suddenly fall gravely ill, at once. If one of them does, the other can consecrate a successor.
It's being conservative, and leaving a lot to God's providence. A good bishop should be holy enough to remember that God is always in control. From what I've heard from his own mouth, Bishop Williamson fits this description. He often reminds us that God is in control. There's nothing wrong with that.