Backup masks are usually carried as insurance against losing a mask, not necessarily because your mask is fogging or being hard to clear.
There are two basic ways to clear: look up and hold mask to forehead to let water leave between the skirt and the upper lip, or look down and let water leave through the purge valve.
Obviously, the looking down method only works if your mask has a purge valve. I'm comfortable with a little water in my mask, and will often clear looking down in my non-purge-valve mask. It leaves water pooled around the nose, which bugs a lot of people. However, it also acts as an incentive to breathe through your mouth, which is the proper way to breathe underwater. Breathing through the nose leads to air bubbles leaving the mask with every exhalation, and every time that happens, another drop of water has a chance to get in the mask.
Masks also often leak because of things caught in the skirt. Run a finger around the entire edge of the mask before you get in to check for hair or a folded over skirt.
If you have to clear all the time, there is a problem somewhere, and you should try to fix it.