The only time double fairs should be done is during a combo imo. So it's not necessary to c-stick both since you should be practically gauranteed the first hit to try it. Trying to shield pressure with double fairs generally isn't very good. A lot of characters can sh nair our of shield after the first hit and reach you, or they can just roll or waveshield in after the first swipe as well. I do, however, use c-stick to space practically every aerial such as single spaced fairs, upairs, dairs, and the second hit of a sh double fair.
So I would say while technically it makes it easier to sh double fair without getting shield grabbed, if your doing double fairs when they are shielding against better players you will get punished for it.