Ireland in 2015 were the last team outside the pair to win the tournament, with France winning in 2016 and 2018 and England taking the other five titles.
England and France have won all three of their matches in this year's competition, with Middleton's side winning by at least 50 points in each of their games against Scotland, Wales and Italy.
"We have to close this gap," Middleton, who will step down after the tournament, told the BBC on Wednesday. "The rate of acceleration and closing the gap will all depend on funding. We have to make these games more competitive.
"We've been in this situation for a long time. We know going back eight years, Ireland were a real force, France, ourselves, and Wales beat an England side in 2015.
"I'm not sure it (Six Nations) can continue in the guise it is now where it's always going to come down to the last game, England and France, because that's not good for anybody."
England, who are aiming for a fifth straight championship, will face France on April 29th at Twickenham.
