When in creative mode (have not tried survival yet) and placing a piston and redstone blocks in this order:

Stone (with lever to activate upper piston as needed)
Piston (facing Down)
Redstone Block
air gap
Piston (facing down)

The lower piston will alwase be actve even tho there is no power going to it when the upper piston extends it remains active but when it retracts from being extended the lower piston quickly retracts and then extends. If this is not a bug sorry.