Simply adding a second engine DOES NOT assure redundancy!
Look at
FAR 23.49 - if a multi-engine ship cannot meet a specified OEI climb profile, then it is constrained to a Vso no greater than 61 kt., same as a single, because is it assumed a forced landing is likely.
Wartime C-47s were often flown at TOGW 20% above the civil limit, and their OEI performance at that weight was nil - the plane was going DOWN. Thus the second engine doubled their risk; they would have been better served by a single R-2800!
Granted, no modern transport is so constrained, but this example serves as a reminder: multiple systems
DO NOT automatically provide redundancy.