Two turkeys of the same size, say 14 lbs, take the same amount of time as a single turkey of 14 lbs............as long as your smoker can maintain your desired chamber temperature and has even heat flow.
A 14 lb turkey is a big heat sink, meaning it will absorb a lot of the available heat energy in the smoker. Two 14 lbs turkeys will absorb twice as much heat. If a smoker can only put out a limited amount of heat, like an electric smoker, the chamber temp could actually be lower than the temperature set on the dial. A lower chamber temp means a longer smoke until the turkeys are done. That's why you ALWAYS use internal temperature of the meat to determine when the turkey has finished cooking.
I don't have a Green Mountain Grill, but it is a pellet smoker with a chamber sensor, meaning the computer will feed fuel to the fire to maintain the desired chamber temperature. You may notice a higher fuel consumption than you are used to, so check your pellet load more often.
Desired smoking temp? To get a beautiful, golden brown bird, moist inside with crispy skin, crispy skin being the trick, 325F to 350F chamber temp does beautifully. Take the coldest part of the bird to 165F internal temp. A bird with nothing in the cavity will take roughly from 10 to 15 minutes per pound, so a 14 lb bird will take anywhere from 2 hrs 15 minutes to 3 hrs 30 minutes. Two birds take the same amount of time as a single bird.
If you stuff the cavity with veggies and fruit, that adds to the mass to be cooked, so weigh the additions before you stick them in the bird. I'm not a fan of putting anything in the cavity but herbs for food safety reasons, but many do put onions, apples, celery, carrots, etc.