Lets assume you're taking "fresh, uninjected" birds, not pre-brined injected birds.
My original brine recipe says "good for brining 12 lbs of poultry." If I have more than that I increase the brine proportionally. In your case you have 28 lbs of poultry, or 2.33 times the amount of poultry. No problem if you just double it as long as the two birds are together in the one cooler.
Many times I've found myself having to use more water than I expected to cover the bird after preparing the brine. I just add more water and don't bother adding more salt or spices. Still turns out delicious. A little salt goes a long way.
Also, since you're not "curing" the bird there's no problem cutting the amount of salt in a brine. All I try to smoke/cook any more at TG are fresh birds, not injected ones. I use approximately half the salt most folks use to brine their birds. My wife is a "super-taster" and complains about turkey being too salty if I use the amount of salt in most commercial or online recipes.
If you are smoking/cooking a pre-brined or injected turkey, DEFINITELY cut the amount of salt by 1/2 or more and cut back on the brining time.