Anybody sitting next to the operator of the vehicle causes inconvenience that may lead to road accidents. Unless, the equipment is designed such that the vehicle controls are within the reach of the operator and that the sitting position of co-riders in noway obstructs operation/control of the vehicle, one should not permit riding on the vehicle.
It is quite common to see persons sitting on either side of the driver of tractors and sometimes on the connecting attachment between the tractor and the trailer, causing inconvenience to the driver. These person apart from causing inconvenience also cause diversion of driver's attention by talking among themselves and with the driver.
We should not add seats or modify the design of the vehicle on our own, unless it is permitted by the supplier. As said above, one should see whether the vehicle can be operated safely by the driver, with the changed design i.e addition of seats. Otherwise, we will be creating hazard for the persons in the factory.
I read sometime ago, that in some places, double riding is a punishable offence. A case study in the form of judgement teaches us that factory management also got responsibility to create awareness among employees about do's and don't's for forklift use/operation.