You have the measurement of a leader. Imagine yourself measuring said leader (his/her height) and then coming up short. This is not good, you think. Leaders need to be at least six feet tall. So, to make the leader tall enough, you attach another head to the top of his/her head.
You have to remember the kun'yomi reading of this kanji, and the part you need to remember is つ. So, think of attaching something to something else. What would you need? You'd need two (つ) things to put together!