Soak the black chickpeas and garbanzo beans in to water overnight. Boil black chickpeas and garbanzo beans together in a pressure cooker for about 3-4 whistle. In a mixing bowl, add boiled chickpeas, tomato, onion, coriander leaves, sprinkle some chaat masala and salt. Mix all and keep it aside. For potato masala, boil and mash the potato, add salt, crushed coriander seeds, chili flakes, coriander leaves and green chutney. Mix all well and Keep it aside.