Saute onion and pepper in butter in large saucepan. Add soups and stir to mix. Thin soup with chicken broth as needed. When hot, add cubed velveeta cheese and stir until melts. Stir in chicken and thin with broth again if needed. Mix chicken mixture with cooked spaghetti. Place in a greased casserole dish. Bake at 350 for 30 minutes.