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Of course your odds of having an engine failure with two engines is
double of what it would be with one, and quadruple with four.

Only approximately. The only reason doubling (or quadrupling) the number of
engines doubles (or quadruples) the chance of an engine failure
(approximately) is that the failure rate is so low. For example, if the
failure rate were 50%, a doubling of that would cause you to expect an
engine to fail each flight (a 100% chance of failure), when in fact the
chance is actually only 75%.

Pete