Agreed, reacting to light-speed attacks makes no sense, whether for people or ships, but I guess it's cinematic.
Sure, reacting to light-speed attacks is perfectly sensible.
Sure, the first sandcaster increases armour by 6 or so, and the second sandcaster increases armour by 1.
The rule is just a cinematic bodge, to make sand gunners have something to do.
Requiring several canisters for each reaction will just force the defender to use just one sandcaster, to get more reactions, preventing a lot more damage per re-load.
But, agreed, your game, your house-rules.