If they are in series, they both "see" the same current. If the curves overlap at that current, you melt both (without considering manufacturers tollerances).
If the curves overlap at that current, I would expect the time constant for one fuse to be different from that of the other. In that case one might melt open first although the other might suffer some sag and change in properties if the current was interrupted before it actually blew.
For identical fuses it is likely that both would melt down, although only one of them might show the deposition of metal on the shell resulting from the arc when the fuse opens.
For a non-glass cartridge fuse, you would likely not be able to see any internal changes to the fuse short of blowing.