Cube your pork belly and pork shoulder, then season with salt, pepper, oregano, and cumin. Toss to combine and mix well.
In a heavy pan able to snugly hold the meat (about 2qt), add 2 tbsp oil and saute the onions until golden and translucent. Add garlic and saute for another minute.
Add the meat, cinnamon, and orange juice. Bring to a boil, stirring occasionally.
Turn the heat all the way down as low as it will go. Cook, stirring once every hour or so, trying to keep the meat as flat and packed down as much as possible.
When the meat is done, remove the cinnamon stick and discard. To make tacos, crisp up the meat in a dry cast iron or nonstick skillet, then serve in doubled up warm tortillas with plenty of onions, cilantro, lime, and salsa.
Notes
Storage: you can save any extra carnitas in an airtight container in the fridge.