As far as I am aware, the top pallet must be under 1/3 of the backrest e.g. If the top pallet is 600mm in height, the bottom 200mm of the pallet, must be supported by the backrest. However, the two pallets must be secured with efficient packaging and support. The minute an operator lifts the pallet, they are responsible. Remember one of the stacking principles, the heaviest must be on the bottom of the two.