Just found this trick that basically upgrades your Fillet/Zinger burger with cheese and bacon for no added cost.
Via the KFC mobile app under Colonels Table, if you order a double combo for $12.95 which gets you 2x Zinger/Fillet burgers, 2x sides & 2x drinks you can get a free upgrade to either the Original or Zinger Bacon & Cheese Burger. Just deselect the two default burger choices and scroll down until you see Original/Zinger Bacon & Cheese Burger. Select those two and the price should remain $12.95.
This might be a price error. Not sure if it will stay around for long.
Those burgers are not listed for that combo for any of the stores I checked. Probably your store messed up their menu somehow (the bacon and cheese burgers are "new" menu items, meant to be $1 more than usual - which is just the same as the regular burgers plus manually adding cheese and bacon extras).