Place tomatillos and Roma tomatoes into a medium saucepan and add enough water just to cover the vegetables. Bring to a boil and boil for 8 to 10 minutes or until the tomatoes are soft. Remove with a slotted spoon and place into a blender. Process until smooth.
In a large deep skillet over medium heat add oil. Add onion and cook until soft. Stir in garlic and cook for 1 minute. Pour the tomato sauce from the blender into the skillet. Add oregano, marjoram, thyme, salt, and pepper. Add the sauce from the chipotle in adobo sauce. Let the sauce simmer until it becomes thick (About 10 minutes). Add the chicken to the pan and toss with the sauce. Cook until heated through.
In a medium saucepan over low heat, heat refried beans. To layer tostadas- Spread a layer of refried beans on the top of a tostada shell. Add chicken mixture, top with queso fresco, lettuce, and avocado.