When you stack 2 sticky pistons facing up and place a block on the top of them, then power them with a redstone torch on the same level as the top piston and with a block above the torch (picture 1), both pistons extend normaly (picture 2).
When you remove the torch now, both pistons retract at the same time, but the lower one will take the upper one with it and the block on top of them will stay where it was as the pistons were extended (picture 3).