Coarsely chop 1/2 small Spanish or sweet onion and 3 large garlic cloves. Add the onion, garlic, 1 cup sour orange juice, 1 tablespoon dried oregano, 1 teaspoon kosher salt, 1/2 teaspoon ground cumin, and 1/4 teaspoon black pepper in a blender or food processor fitted with the blade attachment. Pulse until the mixture is mostly blended together and still a little bit chunky, 3 to 4 (1-second) pulses.
Pat 2 (about 1-pound) pork tenderloins dry with paper towels. Place in a large zip-top bag. Pour the marinade into the bag and massage the pork with the marinade so that the pork is evenly coated. Seal the bag and refrigerate for at least 4 hours or up to overnight.
Let the pork sit at room temperature for at least 10 minutes. Meanwhile, heat an air fryer to 400ºF for about 5 minutes.
Coat the air fryer basket with cooking spray. Remove the pork tenderloins from the bag (air fry in 2 batches if needed), letting the excess marinade drip back into the bag, and place in the air fryer basket; discard the remaining marinade. Air fry for 15 minutes. Flip the pork and continue air frying until an instant-read thermometer inserted into the center of the thickest part registers at least 145˚F, 4 to 5 minutes more.
Transfer the pork to a clean cutting board and let rest for about 5 minutes. Cut crosswise into slices of desired thickness.