Strap one of the rucksacks to the tank and the other across the panniers behind the pillion. Passengers wearing rucksacks is a bad move in my opinion. It raises the CoG, the passenger will get more uncomfortable/numb bum, and the possibility of injury rises in a crash as the road grabs the bag and causes them to roll.
If space is tight bin one of the rucksacks and buy some cheap clothes/toiletries etc when you get over there.