The emergency engine could be massively less sophisticated because of its rare and very short term utilsation - like a cruise missile jet. It could be cheap, noisy, polluting, dreadfully inefficient but above light. The problem would be: would all of the savings from buying and running only one super-expensive high-tech motor be entirely offset by the cost of carrying the extra engine for the life of the aircraft? If not, then to me it's a no-brainer: stop putting two identical (half-power) engines on plane when a full power one is now feasible for everything except A380.
Or... the principal in-flight engine in the tail section as DC-10 / L-1011 and a couple of smaller podded jets on the wings that would only be used for take-off / climb (performance) and landing (security) phases.
Probably still easier and cheaper to go with a conventional twin!!!
