HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

How is that possible? In one game, I play, and in the second, I immediately look at the saves :D?

Well, to each their own. Everyone judges according to their own level of… let’s say, “experience.” And in general, I played 2 vs. 2 in one window, and since they take a long time to make their moves, I played 1 vs. 1 in the second window.