Still trying to figure out the cooking time for two turkeys simultaneously. I was going to get one big turkey but the deal on the smaller turkeys was too good to pass up.
Use the time suggested on the package for the smallest bird, add about 25% for oven variations, etc. Then, run out and buy a good instant read meat thermometer. Every bird, every recipe, every oven is going to toss in variables. And remember, when you pull it, cover the bird with two sheets of foil, and internal temps will go up about 10 degrees over the next 20-30 minutes.
Your cook time should be figured only on the weight of the larger turkey. Not a combined weight. Take the smaller turkey out of the oven probably ten to 15 minutes before the larger one.
Personally I use aluminum foil to tent over the turkey during most of the cooking, then removing the foil during the last ten minutes or so for browning of the skin.