It was 1.8 C difference in skin temperature, not core body temperature. As you note, 1.8 C would be massive for core temp.
Wearable thermometer patches attached to each man’s head, chest, hands, feet, and legs recorded body temperature at five-minute intervals, nonstop, for the entire 10 days of the expedition.
I'll argue that, if it got down to the sharp edge of survival's knife, only the 2-degree warmer twin would come home. 2 degrees C (3 F) is palpably warmer.
That being said, if a 2-degree dip in temp would kill you, you are already praying for Ernest Shackleton's leadership.
Wearable thermometer patches attached to each man’s head, chest, hands, feet, and legs recorded body temperature at five-minute intervals, nonstop, for the entire 10 days of the expedition.