What I would do is to take a tape measure to the store and measure a 14 pound turkey. If two will fit your in luck. One thing to think about is, are there two temp probes, one for each turkey. I assume that you will remove the top rack and do both on the bottom one. I only do one large turkey and I use a "Turkey Done Right" holder that suspends the turkey, breast side down and a temp probe in the thicker part of the thigh. I have had great results with this set up and the breast meat stays moist. I set the rack in a tin foil flat pan with liquid in it to provide moisture to the cook and catch the drippings. Thanks for joining the form, I am sure you will get some good info from our members.
