You are correct except that the design engineers don't (or shouldn't!) assume anything. I was an engineer on a number of military contracts and basic conditions including the number of personnel, their MOSs (and additional specialty training) and level of experience are always specified in the SOW (Statement of Work) for the contract. If the engineers determine that more personnel are needed or if they need additional training or other special requirements such as minimum strength requirement or a height limitation then it's one of their jobs to bring those to the attention of the contracting agency and they implement those or make other changes to avoid those requirements. I personally upset a lot of apple carts by asking for, and getting, additional training, safety equipment, tools, manpower, etc on several major military programs. I wasn't willing to put people at risk un-necessarily which frequently didn't make me popular with program management!
But to get back to this case, the military has always used both a driver and an assistant driver, even since before they started using motor vehicles, so the equipment design and operating procedures were implemented with the knowledge that two people were available. We operate differently so it's not inappropriate that we change the equipment and procedures to reflect that (keeping safety in mind, of course!)
