Although chili powder and cumin are found in both spice mixes, usually there is a larger amount of chili powder in taco seasoning, and cumin is more prevalent in a fajita blend. Additionally, most store-bought fajita mixes add citric acid to replicate the acidity that is typically found in fajita dishes. You can replicate this acidity at home simply by squeezing in some fresh lime juice to the meat marinade, or adding a squeeze of lime at the end of cooking.