When an arcing fault occurs downstream from two 100A fuses in parallel, can the fuses modelled by a single 100A fuse by shifting the current axis to the right by a factor of two to represent the time-current curve for the two parallel 100A fuses ?
The above assumes that the current divides equally between the two 100A parallel fuses.

What voltage? Medium Voltage fuses have a parallel mounting for larger ratings. i.e. 540E which is made up of 2 - 300E fuses in parallel. 540E = (2 x 300E) x 90% The curve is specific for a 540. I did not overlay a shifted 300E TCC with a 540E but it looks like the shapes are similar.

In the U.S. we don't parallel LV fuses (except perhaps cable limiters on networks)

There is a small problem with this. The average fuse rating doubles but the fuse accuracy goes down. Fuse curves show a range of values to accomodate a +/-% factor. If you parallel them, you might get a batch which are both on the extreme to either the positive (bad from an arc flash point of view) or negative side.