Keep in mind that using either quilts or sleeping bags below -10°F requires some experience using them and requires well-thought-out insulation for your head and face and a very efficient sleeping pad insulator underneath you.
See more information about sleeping pads. When camping in the cold, bring warm baselayers, plenty of insulating clothing, and hot food. Make sure you have quick access to your vehicle if something should go wrong, and inform friends or family of your whereabouts.
The bottom line is that you can stack bags but it is warmer and lighter if you have an appropriately rated bag.
For careful use on an ad hoc basis stacking two bags, one as a bag and one as a quilt over it (to get the full loft and avoid compression) can work.