I wonder if anyone has realistic figures for this, but if the chance of an engine failure on a given flight is one in 100,000, and the chance of a single fault failing both engines is one in a billion, this would mean that one of every 10,000 engine failures is a dual failure. In that scenario, when the left engine failed, the odds of the right engine failing would immediately jump by a factor of 10 (from one in 100,000 to one in 10,000).