Place the chicken breasts in the bottom of a 6-quart slow cooker.
Pour the cream of chicken soup, cream of mushroom soup, and undrained diced tomatoes over the chicken. Sprinkle with garlic powder, onion powder, and black pepper.
Cover and cook on LOW for 4-6 hours or HIGH for 3-4 hours, until the chicken is tender and shreds easily.
Remove the chicken from the pot and shred it using two forks. While the chicken is out, whisk the cubed cream cheese into the hot sauce in the slow cooker until melted and smooth.
Return the shredded chicken to the slow cooker and stir to coat.
While the sauce finishes, cook the spaghetti on the stovetop according to package directions until al dente. Drain well.
Add the cooked spaghetti and 1.5 cups of the shredded cheese to the slow cooker. Toss well until the pasta is fully coated in the creamy sauce.
Top with the remaining 0.5 cup of cheese, cover, and let sit for 5-10 minutes until the cheese is melted and bubbly.
Garnish with fresh chopped chives and serve immediately.