In a mixing bowl, whisk together 1/2 cup honey mustard dressing and 1/2 cup buffalo wing sauce until smooth.
Place the chicken breasts in a gallon zip-top bag and gently pound them with a meat mallet or rolling pin to an even thickness.
Pour the prepared marinade into the bag with the chicken, seal the bag, remove excess air, and refrigerate for 4 hours to overnight.
Remove chicken from the marinade, discard the used marinade, and preheat a grill or grill pan to medium-high.
Grill the chicken 6 to 8 minutes per side, or until an instant-read thermometer registers 165°F (74°C), about 12 to 15 minutes total depending on thickness.
Transfer to a plate, let rest a few minutes, then slice or serve whole.