Whisk together 1/2 cup honey mustard dressing and 1/2 cup buffalo wing sauce in a mixing bowl until smooth.
Place the chicken breasts in a gallon-size resealable bag and gently pound them with a meat mallet or rolling pin to an even thickness.
Pour the marinade into the bag with the chicken, seal the bag removing excess air, and refrigerate for 4 hours or up to overnight.
Preheat a grill or grill pan to medium-high heat. Remove chicken from the marinade and discard the used marinade.
Grill the chicken until cooked through, about 12 to 15 minutes total, turning once, or until an instant-read thermometer registers 165°F (74°C).
Let the chicken rest a few minutes before slicing and serving.
