Preheat oven to 425. Prick each sweet potato twice and bake for 45 minutes to an hour.
Meanwhile, prepare the vegan tzatziki and chili. Blend all ingredients for vegan tzatziki in a high speed blender and set aside for later. Rinse and drain red lentils and set aside. Do the same for the green lentils and set them aside separately. Sauté onion and garlic in a large pot over medium high until browned and fragrant, about 5 minutes. Add chopped bell pepper, tomatoes, and tomato paste and continue to simmer for 5-10 more minutes. Add the vegetable broth, cumin, chili powder, black pepper and the green or brown lentils to the pot and bring to a boil. Once boiling, reduce heat to medium and simmer lentils for 15 minutes.
Add the red lentils to the pot and simmer for 15 more minutes. Once lentils are tender, add drained chickpeas, salt, sumac, and most of chopped cilantro, reserving some for topping. Cook for 5 minutes and remove from heat.
Remove sweet potatoes from oven. Once cool enough to handle, cut a slit down the potatoes and stuff each potato with a large scoop of chili. Top each potato with a scoop of tzatziki, pomegranate seeds if desired, and remaining cilantro. Serve immediately.