Bring 2 cups water to a boil in a medium saucepan over medium-high heat. Add guajillo and pasilla chiles. Reduce heat to medium-low; simmer until chiles soften, about 10 minutes. Transfer chiles to a plate; remove and discard stems and seeds. Discard water.

Strain pineapple; reserve juice and pineapple chunks separately. Coarsely chop ½ onion; slice remaining ½ onion and set aside.

Combine guajillo chiles, pasilla chiles, pineapple juice, chopped ½ onion, vinegar, garlic, chipotle, and cumin in the bowl of a food processor; purée until smooth, about 2 minutes. Transfer to a saucepan over medium-high heat; bring to a boil. Cook until chile paste loses raw onion taste, about 2 minutes. Season with adobo; cool.

Combine pork cubes, cooled chile paste, and pineapple chunks in a large glass or ceramic bowl with a lid or in a large resealable plastic bag; toss to coat (cover the bowl or squeeze out excess air and seal the bag). Marinate in the refrigerator for 2 to 24 hours.

Heat oil in large skillet over medium-high heat. Remove pork and pineapple from marinade and shake off excess. Discard marinade. Cook pork and pineapple in batches in the skillet until dark golden brown on all sides and cooked through, about 15 minutes; transfer to a large serving plate.