A few of comments.
First, the hypothermia grading scales that use the terms “mild”, “moderate”, and “severe” hypothermia are of limited usefulness because there are multiple different schemes that define these levels of hypothermia in ways that are inconsistent. Grading scales that are based on core temperature are also of limited usefulness in the field because that often cannot be accurately measured.
Most professionals involved in the treatment of hypothermia now use the Swiss 5 tier rating scale for hypothermia which is based on easily observable criteria such as the presence or absence of a confused mental state, unconsciousness, the presence or absence of shivering, and the presence or absence of vital signs.
The scheme is as follows: HT-1: victim is conscious with clear mentation, is shivering, and has vital signs. HT-2: victim is conscious but has an impaired level of consciousness, is not shivering, and has vital signs. HT-3: victim is unconscious, is not shivering, but has vital signs. HT-4: victim is unconscious, is not shivering, and has no discernible vital signs. HT-5: basically dead. Patient is without vital signs and cannot be resucitated due to imcompressibility of the chest wall and (usually) severe hyperkalemia.
In the Swiss scheme the boundary between HT-1 and HT-2 usually occurs at a core temperature of around 90 degrees F or slightly less, but this can vary between different age groups and individuals. But if you look at the older grading systems, this degree of hypothermia is considered to be the boundary between “mild” and “moderate” in some, in the middle of the “moderate” range in others, and at the boundary between the “moderate” and “severe” range in others. So saying a person has “severe” hypothermia is somewhat ambiguous unless you define the criteria upon which that is based. The chart on page 15 of this pdf describing the Swiss rating system tells the story:
Second, I did not mention this before but will do so now. This is an extract from princeton.edu article you posted earlier:
“No matter how cold, patients can still internally rewarm themselves much more efficiently than any external rewarming.”
That is just plain wrong. It is so wrong that it suggests that the people who prepared that article have never actually seen or treated an individual with very severe hypothermia. If it were true, all that would have to be done in the hospital to treat those individuals would be to put them in a sleeping bag in a warm room and give them a bowl of warm broth, in other words passive rewarming. There would be no need for the entire panoply of active rewarming measures, one or more of which are often needed to rewarm a critically hypothermic individual.
I am talking about measures such as airway rewarming with warm, humidified oxygen-rich air delivered by nasal tube, mask, or even mechanical ventilator, external circulating warm air rewarming with a Bair Hugger or similar device, the administration of warmed intravenous fluids, body cavity irrigation of the pleural space or peritoneal cavity using warmed isotonic fluids delivered and exchanged through either thoracostomy tubes or peritoneal dialysis catheters, or active blood rewarming using a hemodialysis machine or ECMO machine. The fact is that one or more of these measures is very commonly necessary because a critically hypothermic individual has often had complete collapse of their thermoregulatory mechanism and are incapable of warming themselves with passive measures.
As for the pros and cons of placing two people in a sleeping bag, if you want to have a rational discussion this article out of NOLS is a reasonable place to start:
The author points out that while for maximal conductive heat transfer bare skin to bare skin contact is best, direct skin contact between victim and rescuer is quite limited so that conductive heat transfer does not amount to much. So bare skin on the part of the rescuer is not necessary and certainly warm, dry clothes should not be withheld from the victim, if available. The author also appears to consider quite dubious the assertion that direct skin warming is bad because it blunts the shivering response. I find that argument nonsensical because if that were so any skin warming measures such as warm compresses or hot water bottles would also be bad.
The value of having another person or persons in a confined space or shelter is that the non-hypothermic rescuers warm the air in it through convection of body heat. An enclosed shelter like a bothy also has the benefit that the air inside of it fairly rapidly becomes warmed and humidifed which reduces the heat loss that would otherwise occur as a result of respiration, which might not be the case if the victim were in a bivy bag.
Again, you use what you have. If you only have a one person bivy sack or sleeping bag that two people can’t possibly get into you are obviously not going to do that. Obviously, you are not going to delay evacuation if available so as to warm someone up in a bivy or bag. That seems to be a completely made-up argument. If you have a two person bothy shelter that is what you are going to use, unless the victim is too unstable to be safely kept in a semi-upright sitting position. Even then the bothy can be used as a small tent to cover a victim lying in a fetal position if the roof is held up with an appropriate length stick or some external cordage.
Putting two people in a bag or bivy may not be the most effective way of providing and external source of warming for a hypothermic individual, but it might be the best available under the circumstances.