Combine chicken, buttermilk, and 1 packet taco seasoning in a resealable plastic bag. Seal and shake, then let marinate in the refrigerator for 8 hours to overnight.
Heat oil in a large skillet over medium-low heat.
Mix together flour and remaining packet taco seasoning in a shallow dish or plate. Remove chicken from marinade; discard marinade. Dredge chicken in flour mixture.
Fry chicken in hot oil on both sides until no longer pink in the center and the juices run clear, 15 to 20 minutes. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C).