Be very very careful with any risk assessment. As i have already said carrying an unsupported pallet on top of one on the forks is inherently dangerous, no matter what the load toilets rolls or wine it will can still shift and under cornering will. In a high volume distribution centre you need to work to best practice, double stacking loads isn't good practice let alone best practice.
Whoever signs off the risk assessment is really putting him or herself in a very risky position should an reportable accident ever occur (and running double stacked pallets it most probarbly will) the signee will be one of those along with the competent person on site and the company MD who has to stand in front of a Judge and try to explain how they decided this was safe