Take both fishes out of the tank for 5-6 hours.
"Store" them separately.
Redecorate the tank.
Introduce the "weak" specimen first.
Let it get familiar with the new surroundings (an hour or two).
Then introduce the dominant specimen.
Chances are this will change the relationship.
Puffers are smart, but not that smart...
Btw; you might wanna extend the periods of time Ive been recommending.
Storing them for 24 hours, then add specimen 1, wait 24 hours, then specimen 2.
Good luck!