This isnt the American Army, and I'm aware of the ranks of generals. However, if two generals were of the same rank, command of a camp is usually conceeded to whoever is in command first, that usually goes for any rank.
edit: And I'm using a very simplified version for a reason, mainly because of the fact that it's a Japanese game. I know there's all sorts of circumstances and whatnot, but the given that it's a video game, and from another country to boot, I doubt those things matter.