It´s a per liter amount! if you develop 2 you have to double the amount of the developer. Usually you have a 8 to 12 gr per liter of thiosulfate. So you have to do a racio for the amout of developer you use.
Thinking in consumption of chemistry by the silver halides is not a good thinking, the other way around is closer, and it would be valid for the other chemical compound of the developer. Don´t think about it, just use the formula correctly with the correct amounts.
by the way, keep the thiosulfate amounts to a minimum possible, it eats away your max density.
Cheers
Rui Lourosa