How exactly does phantom's carte noir work?

The skill description reads "Upon a successful critical attack, card appears at 100% chance."

When using mille, what exactly counts as a successful critical attack? I always thought that each line of damage from mille that crits would proc carte noir but is it actually 2lines from mille = 1 hit so only 1 carte noir proc?

It is per attack as far as I know, so 2 hits of Mille = 1 attack = 1 proc of carte noir.