Roasting multiple turkeys (cooking time estimation)
This looks like an excellent answer to almost exactly the same question, found here.

Tuxman
Nov. 26, 2013 11:01 am


  I am trying to get a clear answer on cooking 2-15 lbs birds in
  different pans/same oven. what would the cooking time, same 20 min per
  lb on 325 degrees? for convection oven?


dae
Nov. 27, 2013 12:43 pm


  @Tuxman: Each bird cooks separately. So it is like cooking one 15 lb
  bird. Start at same time. If one is bigger, it will take longer than
  the other. Allow max space between them. Put a digital probe deep in
  the thigh of the smaller one (avoid contact with bone), and take out
  when it gets to 165 take it out. Put probe in larger one, into thigh,
  take out when it gets to 165.