Health Boost II is the middle step of the health line, giving one more bar segment for two points. It needs level 2 and the first Health Boost.
Health Boost II sits at position 6 of 18 in the Combat tree. Unlocking Health Boost II takes 2 skill points. You need level 2 and Health Boost I before you can buy it. Jacob and Evie each have their own skill tree, so you buy Health Boost II once for each twin.