Oops! I think they aren't pre-cooked. Just specially prepared. Anyway the instructions say to cook at 400 degrees for 3 hours. I have already cooked them individually before with incredible results but now I need to cook them together. How much ADDITIONAL time do I need if any?
Oh, okay, I see. They're not pre-cooked, probably just pre-seasoned or injected or something.
They do not need additional time. If anything, at 400 degrees they may need less time. Watch for wisps of black snoke around the edges of your oven door. :D
You might submit your question on the turkeys to Foodnetwork.com. I'd think one of the chefs over there would know the answer because I've never tried cooking two birds at once.
Put a meat thermometer in the biggest one. You should be good at around 170 degrees internal temp.
I always used to do two turkeys when I did holidays. I'd cook one the night before, slice it up & put it on a microwaveable platter. I'd cook the second one on the holiday & throwing the drumsticks from the first one in with it for about 20 minutes / half hour to warm them up.
It's a good way to get extra stuffing & gravy, plus you're dealing with younger, more tender birds. Like you, I go with high heat. I use the roaster cover, plus foil & baste, starting around an hour after I put the turkey in the oven.
If you're going to try cooking both at the same time, I'm thinking you'd need to add about an extra half hour to an hour. A meat thermometer would be your best guide. Guests are generally patient if the meal needs extra time, so I'd add the half hour into my calculations & if it takes a little longer, so be it.