Place 2 chicken breasts in an instant pressure cooker with 1 cup of dressing. Cook on high pressure for 20 minutes or until chicken reaches over 165°F. Use two forks and shred the chicken in the marinade and cool.
Cook the pasta according to package instructions, cool and toss with the remaining dressing, chicken, tomatoes, spinach, onion, and mayonnaise. Serve cold or chilled.