I would say this is not necessariy the case.
It certainly depends upon the exact circumstances.
Assuming the load is evenly distributed, each fuse will carry half the overload current. Thus the first fuse to blow may blow more slowly.
One would have to look at the curves, but I rather doubt that it would be significantly, if at all, slower - since half the overload current going through the half-rated fuse would represent exactly the same percentage of I
n (i.e. the same proportionate degree of over-current) in both cases.
The overload current will then be fully carried by the second fuse until that blows.
True, but that fuse should blow very quickly, since a current which overloaded the circuit as a whole would be way over the I
n of that second fuse.
The total energy let through by the two fuses with the delay is likely to be greater than would be let through by one fuse.
Probably, yes (as you say, one would have to look at the curves and do some sums) but, as above, I suspect only slightly so.
However, it is in situations of seriously
unevenly distributed (over)loads (the theoretical Achilles heel of a ring final) that the dual fuses would be likely to make things safer. Although, again, the two fuses would have to blow 'in turn', I suspect that dual fusing might win in this situation - but I'd have to do some work to be sure!
In any event, as I said, the main (potentially serious) hazards of dual fusing are nothing to do with cable protection or let-through energy but, rather, to the risk of people finding themselves unexpectedly working on live circuits.
Kind Regards, John.