I have a lot of time on the PA-31-300/310/325/350/P.... never has it been an issue to roll in any direction when adding power on both engines. IF, and only IF, you were below the red line on the airspeed indicator with the critical engine failed, would you see a roll toward the critical engine when power was added. But if both engines were running you went straight ahead.
Without more information than has been provided here, I cannot hypothesize why this aircraft, on this day, rolled right when power was added to both engines.