Manufacturers have done everything they could to maximize the amount of safety that goes into their products. But, injuries can still persist as no matter how safe something is, humans can manage to screw it up. This is especially true when talking about younger people as they are simply too ignorant to know otherwise (with age comes wisdom after all). Because of this, you may see some manufacturers only recommend one child jumping at a time on their model and it has nothing to do with the weight limit.
The chances of injury are greater with two jumpers as collisions can happen and there is less free space for them to jump around in. Clearly, you are the supervisor and you can make the final call but for precaution, you may want to adhere to the advice here.