Pretty fascinating. Imagine the girls snorkeling. Since they share a circulatory system either of the girls could close off her airways and yet the other could supply enough oxygen to support life. The girls have a very small chance of choking to death with such redundancy.
I don't recall seeing anything about sleeping. They have to cooperate to walk, so the twins must be sedentary when either sleeps. Can they sleep different hours?
I sometimes suffer a sudden and relatively violent sneeze. That could be interesting to find out what happens when one of them sneezes. I mean no disrespect to the twins, but I couldn't help thinking of Tim Conway.