Add chicken breasts to the crockpot.
Pour in chicken broth and crushed tomatoes.
Add chopped onion, minced garlic, Italian seasoning, salt, and black pepper.
Cover and cook on low for 4 hours until the chicken is tender.
Remove the chicken from the crockpot and shred it using two forks.
Return the shredded chicken back to the crockpot.
Add the pasta and cook for another 15 minutes until pasta is tender.
Stir in parmesan cheese and mozzarella cheese until melted.
Taste and adjust seasoning if needed.
Garnish with fresh basil and serve hot.