Final answer:
The duration of the twins' surgery is not provided in the given information.
Step-by-step explanation:
The duration of the twins' surgery is not mentioned in the given information. The passages provided discuss various cases where twins have been studied for research purposes, but it does not specify the length of their surgeries. Therefore, we cannot determine how long the twins' surgery lasted based on the given information.
Instead, these references discuss different aspects of twins in medical and scientific studies. For instance, Dr. Edward's reference highlights the long hours of surgery he performed during wartime without mentioning specific cases. Another reference discusses the long-term study of brain development in rare twins. The case of the separated twins from 1968 illustrates the use of twins in behavioral studies. Lastly, the Kelly twins' space study underlines the use of identical twins to understand the impact of environmental factors on human health, particularly in space travel.