The problem with these plugs is that they have a shield that extends down almost to the electrode, and that shield is the "2nd piece." On high mileage plugs it often adheres to the outside of the spark plug hole so that when you turn the plug to remove it the shield breaks loose and all you remove is the plug itself. The shield is still stuck in there around the outside.
Ford has also come out with an improved version of the 2 piece plug that is supposed to hold up better and not let the shield break loose. Always use anti-seize on the outside of the shield when you install the plug.
On Mustangs (4.6L engine) they recommend spraying PB blaster or something similar around the plug base before removal (cold engine). Then carefully turn the plug a little bit, and spray the PB blaster again. Follow this process several times and little by little let the PB blaster get between the shield and the head so that it breaks the bond.