When I do this, I first (i) spatchcock and then second (ii) separate the legs and thighs (i.e. dark meat) from the legless breast (sometimes referred to as a turkey crown).
Then I cook the white meat on one grate (usually the top) and the dark on the other grate. Allows you to cook the dark and white for different times and to different temperatures. Which is the whole ballgame when cooking turkeys.
All the various methods are trying to do the same thing. Cook the breast right (moist and juicy at 157F) but also get tender dark meat at 175-180F. To me, cooking separately is the obvious easiest way to do that.