Comment made in training course dealing with high altitude jet upsets - the first upset typical results in a loss of X (1-3K?) feet. The second upset results in the average loss of Y (7K?) feet.
Scenario - upset one starts. Crew is startled. Rocking and rolling, buffet, etc. Nose drops and altitude loss starts. Plane starts flying again. So far, so good.
Then the crew tries to minimize the altitude loss and the second upset starts. A perfectly flying wing, well below typical flying speed and typical AOA, is exposed to an increase in AOA/G(BOAC's point) in an attempt to reduce the descent rate. Critical AOA is reached very quickly with different results from a low altitude/high AOA event. Bam, upset 2 is entered.
The course mentioned the major injuries, and a/c damage, result from getting into the second upset. One is a basic a 1G event, while the other is an 'accelerated' stall event. As BOAC alluded to, they can be interesting as heck in fighters but they're a bad place to explore in a jet liner.