redstone dust that powers 2 pistons stacked on each other, will power either the top or bottom piston depending on the direction it is facing (North, South, East, West)

 this is not an issue if you use repeaters

However if the pistons face down, the lowest piston will extend as well as the top piston causing a double extender but not retracter. But that is probably due to block updates mechanics that make "jeb doors" possible.