Scavenger II boosts your haul of the rarer crafting resources. It needs level 2 and Scavenger I.
Scavenger II sits at position 12 of 18 in the Ecosystem tree. Unlocking Scavenger II takes 2 skill points. You need level 2 and Scavenger I before you can buy it. Jacob and Evie each have their own skill tree, so you buy Scavenger II once for each twin.