Shvut de'shevut (a restriction on a restriction) is a situation that arises when considering an action. It is not a directive. It doesn't mean you can violate one D'Rabbanan if it violates another D'Rabbanan too.
It means that a certain act can be permitted (under certain or all circumstances) if it is two steps removed from a D'Oraisa violation, and not just one step away.
Asking a gentile to carry a sefer Torah on Shabbos through a private backyard without an eiruv, into a nearby shul.
Asking a gentile to carry is a D'Rabbanan. The backyard is not a "reshus harabim" so it is a D'Rabbanan violation to carry there on Shabbos. The situation we created here is two steps removed from the D'Oraisa of a Jew carrying in a reshus harabim. (It's a gentile carrying in a reshus hayachid for the sake of Jews)
Asking (hinting) to a gentile to turn the light bulb off so the baby can sleep. Asking him is a shevut. Turning off the light bulb is a shevut (no charcoal was created).
Generally, a shevut d'shevut is permitted for a mitzvah purpose or other important concern under important circumstances.
A gezeirah l'gezeirah (decree on top of a decree) is usually something we do not make.
If a gentile turns on the light in a room on Shabbos for his own reading benefit, we may ask him to leave the light on. We do not decree that we may not use the light. Such a decree would be because we fear that were we to get used to asking him to leave the light on, we would eventually ask him to turn it on for us.
But, we do not make such a decree, because even if we fell into asking the gentile to turn it on, that would still only be a D'Rabbanan.
However, we do make a gezeirah on a gezeirah by milk and meat. Cold chicken may not be eaten with a cold glass of milk, even though the real prohibition is a steak cooked in milk. This rare case is because the Sages saw that way leads onto way when it comes to eating food.