Place 6 tortillas in the bottom, overlapping as needed. Top with one jar of green enchilada sauce, spreading out to the edges. Place on top 1/2 of the chicken, 1/3 of the cheese and 1/2 of the onion. Layer on top 6 more tortillas, followed by the red enchilada sauce and then the remaining chicken, 1/3 of the cheese and remaining 1/2 of the onion. Top again with 6 more tortillas, the other jar of green enchilada sauce and the remaining cheese.