I tried to research this and had no luck. I certainly don't remember it ever happening. When was the last time a team led the nation in total defense in consecutive years? The Hokies are number 1 right now and they have two very tough games left so there's a TON of work left to do to maintain that ranking. But if they are successful, will they be the first team to ever do that in consecutive years?
And before you dwell on that question, think about this - the Hokies were 12th in total defense following the Boston College game. They went from 12th to 1st playing Southern Miss, Clemson, Miami and Kent State. Wow.