The newly hired pilot was being trained for a First Officer position in the twin-engine airplane by one of the operator's trainer pilots. After accomplishing air work, the flight returned to practice landings and takeoffs. According to both pilots, the first approach was normal, and the airplane made a normal touchdown, just slightly right of the runway centerline. The trainee pilot stated that after touchdown, the airplane began drifting further to the right, and that he was unable to correct that deviation. The trainer pilot reported that when directional control became unstable, he told the trainee "I have the controls". The trainer pilot observed that the trainee released his hands from the engine power levers and the yoke, but the airplane continued to deviate to the right, contrary to his control inputs. He added asymmetric power to the engines to help regain the centerline, without success. The trainer pilot realized that based on his inputs, the airplane should have returned to the centerline but did not, and that the airplane was behaving as if the trainee pilot was applying some braking. At that point, the trainer pilot "clearly shouted a second time" for the trainee to release the controls. The airplane then exited the right side of the runway and collided with a ditch. During that rollout, the trainer pilot used the brakes, which he reported applied and released normally. The trainer pilot reported that it was his opinion that the trainee pilot's feet were either on the brakes or interfered with his brake and/or rudder application attempts. The trainee reported that he could not recall the position of his feet after he relinquished airplane control to the trainer pilot. The airplane sustained substantial damage to the right wing. The pilot reported that there were no mechanical failures or malfunctions with the airplane that would have precluded normal operation.