Re-read my posts and the included and linked regulations.
Q. As a single driver, why can't you sleep in the rear seat area of your pickup and log I as sleep time?
A. As a single driver (no co-driver) you can sleep in the rear seat, but you cannot log it as "sleeper berth" if the space does not conform to the regulations. You can log it as "off-duty" because the vehicle is not moving as is noted in the above posts. However, that is not the question you started with. Your initial question was for two drivers, so that is the question that was answered.
Q. No one is driving the truck, so why do you need a safety net? Why does the sleeper need to be 78" long? And the need for safety door releases, what is a matter with what the truck has, all are in reach for an emergence?
A. You don't need any of this if you drive the truck as the sole driver, sleep in the back and log it as off-duty rather than sleeper berth. You do need it if you want to create a conforming sleeper. Of course you can sleep in the bed of the truck, on a trailer, on the ground, in a tree, etc., or not sleep at all and log it as off-duty.
Also, you can have two drivers and both can sleep in a parked commercial motor vehicle and log it as "off-duty". However, as soon as that vehicle starts moving, one driver is "driving" and the other is "on-duty" if there is no conforming sleeper.