I'm trying to design a realistic genetically engineered nanotech super-soldier with a full set of redundant organs. How much more interior volume and thus height/weight/muscle mass would a human body need to support 2 hearts, 3 lungs, 2 stomachs, 3 kidneys, 2 livers, 2 sets of large and small intestines, 2 esophagi/trachea, and 2 sets of certain major bones (femur, spine, humerus etc.)? Additionally, could a human brain be "taken apart" and redistributed throughout the body while still functioning? This super soldier should be designed with as many safeguards and redundancies as possible.
Also, this bio-engineered human needs to still look... human. Is it possible to accommodate for all these changes with only an increase in height and muscle mass, or is it inevitable that anyone with this much modification will start to look like a walking tank made of meat?
The extra weight shouldn't be too bad. From Quora:
Approximate body weight distribution for a lean adult:
Bone: men 15%; women 10%
Muscle: men 45%; women 37%
Organs: men and women: 25%
Fat: men 15%; women 28%
So you're looking at adding 25-40% weight, depending on how many of the bones you replace.
However, the brain is an issue. You cannot distribute the brain without drastically changing its functionality. Neuron impulses are actually quite slow (120m/s at the highest), so moving parts of the brain to different areas actually adds a non-trivial latency in communication which would drastically change the way the brain operates.
For an interesting data-point, we actually do quite a bit of processing in the spinal column itself, especially for walking. It's closer to the rest of the body, so the delays are lower. It turns out that if you bump your left hand into something while walking, your right foot will have adjusted its gait before your brain stem has even processed the impact!
As for the shape of your human, the real limiting factor is going to be useful redundancy. I can carry 100 eggs in a basket, but I don't have redundancy if I break them all at once. An extra femur is going to be... well... an extra bone in the leg. If something was breaking leg bones before, it now gets to break twice as many. You're likely going to have to do some substantial restructuring to make that redundancy useful.
You don't need full sized duplicate organs if they are just for emergency redundancy. The liver, for example, can function for a time with a much smaller size, so you could have a smaller accessory lobe somewhere else. But the problem with sudden traumatic liver loss isn't the actual loss of the liver, it is the extreme hemorrhage because of how vascular the liver is.
You could also have collapsed lungs in reserve, but it is hard to imagine where they would be other than in the chest cavity, which is already occupied with the regular lungs. Penetrating chest trauma, chemical attack, or explosive overpressure would affect all the lungs in that area. But if you could somehow collapse the "in use" lungs and inflate a pristine lung in it's place you may be able to mitigate a lot of the damage for a short time.
Back-up hearts distributed elsewhere (such as the pelvis) could work and also wouldn't need to be as large as the primary heart (if it was intended for just a short time before exhaustion). But if you want redundancy for primary heart loss you would need one for pulmonary perfusion as well as systemic perfusion since the primary heart is hooked up to both vascular systems.
What you really need are collateral arteries that can open up to bypass a damaged main artery, like a cut carotid. The drop in blood pressure to the brain causes you to pass out. Anything that could keep pressure up in the brain would allow for continued function, at least for a short while.
You don't really need a back-up kidney in the short term, but you can stick one anywhere, since transplanted kidneys can function anywhere they can get blood (and a drain for urine). Same with back-up intestines. You don't even NEED intestines if you can get all your nutrients intravenously (Total Parenteral Nutrition). The big issue is intestinal injury dumping gut bacteria into the belly, causing sepsis which can kill you pretty quick.
So, in short, it is pretty hard to redistribute or reorganize human anatomy but still retain the basic shape if your goal is combat redundancy. Getting shot in the belly will rupture both sets of intestines. Getting stabbed in the chest will pierce all the lungs there (and you need the rigidity of the ribcage and the diaphragm for lungs to work). A duplicate vascular system would be of benefit, but it would be a neat trick to get it to function properly to allow for bypassing injured areas but still connect in order to perfuse the smaller vessels. You COULD have a collapsed lung that gets opened up in the event the main lungs collapse, as that would just require a limited set of tracheal and vascular branching, but this is a pretty niche use case since the torso is so easy to armor already. Duplicate kidneys, liver, and other organs (like the thyroid, adrenals, pancreas, etc) are either unnecessary (since there is already a second copy in the body) or the immediate loss isn't important. Probably your best option is splitting up the liver into smaller lobes spread across the abdomen. The function shouldn't be impaired so long as the total mass is the same and having smaller sub-livers will limit a major source of abdominal traumatic hemorrhage. One lobe will still need to be near the stomach in order to supply the gallbladder with bile.
When a soldier is shot, the cause of death isn't usually damage to a specific organ, it's from the of trauma caused by the loss of all blood pressure, the shock to the rest of the body from suddenly limited oxygen supply, and bleeding out through the wound.
If you were trying to make a supersoldier, focus on mitigation of injuries. Blood vessels that can constrict to limit bleeding, extra bone and muscle mass to absorb impact from bullets and shockwaves. Less pain and panic response so they keep their cool under pressure.
Put simply, it doesn't make too much sense to double up on internal organs. They need to get blood to them, and a gunshot wound stops that whether or not there's more organs.