Preheat the oven to 400 degrees Fahrenheit.
Begin by preparing the fajita seasoning. Mix the chili powder, paprika, garlic powder, onion powder, cumin salt, and black pepper in a small bowl. Stir together with a small fork. Set aside.
Prepare the veggies. Remove the papery outer layer and slice the onions. Remove the top of the bell peppers and empty out the seeds. Cut in half and remove the membranes, then slice the peppers into strips. Set aside the onions and peppers.
Slice the steak into thin strips. Cut them in half if they are too long to be eaten easily.
Cut the chicken breasts into strips. If using chicken tenders, you will most likely need to cut them in half.
Divide the yellow onion and bell pepper mixture in half.
Add the steak and half of the veggies to one bowl.
Toss the steak and veggies with 1 tbsp of fajita seasoning and 1 tbsp of olive oil. You can use a wooden spoon or your hands.
Place the chicken and the rest of the veggies in the other bowl. Sprinkle with 1 tbsp of fajita seasoning and 1 tbsp oil and mix to combine.
Line the baking sheet with aluminum foil.
Spread the steak fajita mixture on one half of the pan in a single layer. Spread the chicken out in a single layer on the other side of the pan.
Squeeze the juice of one lime over the fajitas.
Cook the fajitas for 15 minutes until the temperature of the steak reads 135-140 degrees on an instant-read thermometer and 165 degrees for the chicken. Remember that the meats may cook at different rates, so check them frequently. I found the chicken needed to stay in the oven a bit longer, but be careful not to over cook the meat or it will be dry.
As you near the end of the fajita cooking time, warm the tortillas. You can do this by placing them in the oven for two minutes or by heating them in the microwave for less than 30 seconds.
Pile the steak fajita mixture or chicken fajita mixture on the warm tortillas. You can squeeze a bit more lime juice over the top and add chopped cilantro if desired. Add optional toppings.