When it says "second target and beyond", is te assumption that you're targeting a second entire NPC, or could it be the second shot on te same target in VATS?

This perk allow each shot on targets after your initial target to do extra damage, but it only applies to a completely new person that you are targeting in VATS, it does not come into effect if you are targeting different parts of one person's body. This does mean however, that if you are in a fight with more than 4 people you can shoot once on each of the 3 initial people and get automatic criticals on every other person that you shoot after that.

from Fallout Wiki

There's a different perk that covers successive shots on the same person, right?

Coordinated, or something? I think that perk applies if you hit the same SPOT on the same person multiple times. I wonder if GunFu would work if you hit Foe #1, then Foe #2 then hit Foe #1 again, then hit Foe #2 again and so on?

Yeah, "Concentrated Fire", I'm pretty sure it's down the Perception tree.

Oops, already covered.