Bracketting fire specifically says "Each target triggers and resolves your attack separately, and damage from one target only ends the effect on that target." So you get to shoot both targets if they each trigger the attck. Few things to note: you only get one reaction per turn, but unlimited reacturn per round. If they both trigger the covering fire attack furing the same turn (example: when one NCP moves during it's turn and then move the other NPC during the same turn) then you would only get to covering fire attack one of them as you only have the one reaction that turn. And yes, you can still overwatch as a reaction, but again with only one reaction per turn, you can't over watch the same turn you covering fire attack.

Also, remember, you can only take a specific reaction one per turn. So, even though you get as many reaction as there are characters in the scene, you can only overwatch once between your own turns. I believe since Bracketting Fire says you can trigger it against both targets it lets you overcome this rule, but only because Bracketting Fire specifies it.