I am an excellent engineer who does a lot of expert witness work, and has done a lot of forklift safety work. The issues with carrying pallets two up are multiple. Firstly if the pallets are of standard height (1000-1300 mm high) the view in the forward direction will be obscured to the degree that driving forward is unsafe. I did an expert witness report of this case where the pedestrian approaching from the other side to the side the driver was observing was killed. Secondly this type of movement tends to result in pallets being stored two high - and that means pedestrians are obscured from view! Thirdly there is the issue of the stability of the stack - does the bottom pallet provide a strong and level surface for the top pallet? If not over time the stack could collapse. Fourthly there is the issue of the stability of the double stack in transport. Often with heavy acceleration or braking the stack can collapse, potentially injuring the driver or pedestrians. I have done two expert witness reports related to these type of events. So a risk assessment needs to be carried out into the whole operation!