Using a rubber spatula, toss chicken with fajita seasoning in a bowl until all the chicken chunks are coated. Set aside.
Heat half of the oil in a cast iron skillet until shimmering.
Add chorizo and the seasoned chicken chunks to the skillet. Cook, stirring often, on medium-high heat until browned and fully cooked (about 6 minutes). Use a wooden spoon or spatula to break up the bulk chorizo meat as it cooks.
While the meat cooks, warm the tortillas (see notes).
Transfer the meat to a bowl and set aside for a moment. Set the skillet back on the stovetop. (Don't clean it, the browned bits from the meat will add flavor to the vegetables).
Add the remaining oil to the skillet, then add onion and pepper slices. Cook on high heat, tossing around a few times, until starting to soften (about 3 minutes).
Top each tortilla with the meat, the vegetables, sour cream, and chopped cilantro. Serve immediately.
