Keep the touch Id as an instance variable in the invisible sprite, make left always 0 and right 1, when you touch it, set the value from the instance variable.
Can you provide more info on the movement, like lets say you touch the left side of the screen, then left player is moving, but how? Do you use a arrow panel where you move the players and tap to select which one you more, of when you just tap then the left size left player moves automatically?