As verbs the difference between doublecross and backstab

is that doublecross is to betray someone by leading them into trap after having gained their trust and led them to believe that they were actually being aided while backstab is to attack someone (especially verbally) unfairly in a deceitful, underhand, or treacherous manner, especially when they're not present in the place or situation that it happens (as if stabbing them in the back) see backbite.

As a noun doublecross

is an instance of betrayal of one who has been led to believe that the betrayer was assisting thim.