When a creature is flanked it is subject to backstabs from any quarter.
However, you might want to make this part of a "tree" for backstabbing.
In other words a player must have the talents "BackStab" and "Guerilla" in order to select the third talent "Sneak Attack"...
Or, conversely, you have access to Sneak Attack at Scoundrel 5, though such sneak attacks still require a dagger as normal.