The reason you have a double brine is that the first one clears out the slime and blood from the fish, and the second one seasons and flavors it. Don't alter the first brine, but you can alter the second as you see fit.
Mix the first brine together and soak the shad fillets in it for 30 minutes, then drain.
Meanwhile, bring the second brine to a simmer, stir well to combine and turn off the heat. Set this in a drafty or cool place to chill it down fast. When the second brine is cool, pour it over the shad and brine for 2 hours.
Drain and rinse off the fillets, then pat dry with a towel. Air dry in a drafty place -- use a fan if need be -- for 2 to 3 hours, or until the meat looks a bit shiny. This is an important step; you are creating a sort of a second skin called a pellicle that is necessary to seal the fillets. If you skip this step, you will have problems with the proteins leaking out from between the flakes of the meat, forming a white icky stuff that will need to be scraped off.
Smoke over alder or hardwoods for 1 to 3 hours, depending on the heat. You want the shad to slowly collect smoke, and cook very slowly. Under no circumstances do you want the heat to get above 200°F. Remove and let cool at room temperature before packing away in the fridge or freezer.
Wood choice is also up to you. For fish, I prefer fruit woods such as apple, or alder, maple or oak.
You can alter your second brine however you like. One fun thing is to increase the amount of sugar/syrup, which will give you a sweet/salty result.
If you want, you can dry your fillets overnight in the fridge, uncovered.
My preferred temperature for this smoke is about 175°F.