DaveReidUK
Not many cases, but the point about the proposal tdracer mentioned is that there would be NO data streaming. This is about copying the data from one recorder to the memory of the other one, and vice versa. Mirroring. All contained within the aircraft. NO streaming. No connection with the outside world. No data transmission costs.
As you say, max. of 16 cases where it might have made a difference, and the one-time cost of designing and implementing the entirely-within-the-aircraft-without-any-data-streaming mirroring solution might well exceed any quantifiable benefits, but I'm surprised it is not discussed more as an ALTERNATIVE to data streaming, which it is NOT.