At which amperage will 2 x 100A fuses mounted onto a dual fuse holder blow?
From your diagram, it looks like the fuses are totally independent and supplying separate circuits, therefore they will both be capable of continuously supplying 100A each.

If one of the circuits draw enough current to blow its fuse, the other fuse would be unaffected.

As @Neil_UK stated in the comments - "They are rated to carry 100A for some 1000s of hours, and will break at some larger current, after some typical time (maybe 200A takes 10s to blow it, depends on the specific fuse, check its specifications)"