load carrying equipment is generally designed to carry a certain type of load depending on the nature of the equipment design. Their design is based on a set configuration calculation.
In the case of a lift truck they are generally designed to carry a single pallet of materials.
If you vary from it's design spec then absolutly, you would need a risk accessment done and appropriate measures would have to be taken to insure the load stability beforehand.
In some cases such as a slipsheet type load, it may be stable enough to carry a double stack load as long as the load stack is stable and can be stacked upon each other without risk of loosing the top load. In any other case where a wooden pallet is being used it would not be recommended to stack them during transport on the lift.