Just build them all with one engine to start with, imagine the fuel savings and other economies that will deliver. If the rationale is to continue flight on one engine for a time which would cover most transit sectors in the world then why do you need two engines to start with?
Because noone is saying one wll never fail( though it doesn't often). It's about building a system where it is extremely unlikely that BOTH will fail. There are indeed single-engine Airline operations and, generally, they deviver high degrees of saftey.
two engines provide even more.