Slice the chicken breast and sirloin steak into thin, uniform strips for even cooking.
In a small bowl, combine the chili powder, cumin, garlic powder, sea salt, and black pepper.
Toss the meat strips with half of the spice blend and the lime juice in a bowl to marinate.
Heat the olive oil in a large cast-iron skillet or grill pan over medium-high heat until shimmering.
Add the chicken and steak strips to the hot skillet, searing until beautifully browned and cooked through.
Remove the meat from the pan and set aside; add the sliced bell peppers and red onions to the same skillet.
Sauté the vegetables with the remaining spice blend for 4-5 minutes until they are tender yet still crisp.
Return the meat to the skillet, tossing everything together to combine the flavors and heat through.
Serve the fajita mixture on a platter topped with fresh avocado slices and chopped cilantro.