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? Most commercial flights take off using reduced thrust settings to start with and then reduce thrust further once in the cruise so that the combined thrust of 2 engines would be roughly equal to one engine operating at higher thrust settings. Redundancy can still be supplied and applied through design for one engine (as it is now for ETOPS). It is nothing new for single engine jet aircraft to operate over large expanses of ocean, it was being done long before ETOPS was ever thought of, with the limiting flight time factor being the engine oil capacity.