You will use your instant-read or leave-in thermometer(s) to determine when they are done. Really, how else are you going to know?

Those pop-up indicators embedded in so many turkeys always let the meat overcook (dry meat, again?). Not all turkeys are shaped alike; not all turkeys cook at exactly the same rate. Whether your turkey is “prebasted” or fresh has an impact on cooking time. It is possible that your turkeys will be done at about the same time. The one sure thing is that they will take much less time to cook than if you were cooking a single 19-pound turkey, where the meat would be much, much thicker than on either of your smaller birds.

Compounding that is the fact that a roaster oven often cooks a turkey in less time than a conventional oven – sometimes as little as half the time. (A roaster oven essentially steams a turkey, and steam transfers heat more efficiently than the drier air of a regular oven.)

Now, we dare you to ask us again to gauge when your turkeys will be done. The roaster oven literature we looked at indicated that you should cook the turkeys for 2 to 2-1/2 hours. They also say that you should not keep opening the lid, as significant heat escapes and you have to add an unknown amount of additional time for it to build up additional heat and finish cooking. With that in mind, it is in your best interests to purchase a probe or leave-in thermometer, so you can monitor the heat of your turkey with the lid in place until it is done. If you were to do that, you would insert the probe in the smaller turkey, and when that one is done, open the lid, remove it, and check the temperature of the larger turkey, which might still need a bit more cooking.