You are correct in thinking that a Concorde with 2 engines out would have much reduced range.........
...and if a double failure like that happened, then there was a very high chance the aircraft would not be able to make it to its destination....however the atlantic route flown was carefully planned so as there was always a suitable airfield within the 2 engine inop range at all times.......Typically, Shannon, Santa Maria (Azores) and Gander.
Ditching any large transport aircraft is very dangerous......ditching a Concorde would likely result in certain death to all involved.
It was progress in technology that caused the demise of the Flight Engineer although of course the airlines didn't complain.