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

As I understand it, the focus can be either on the initiator of the exchange (as in WoG and SoD without a mod) or on the second (right, "passive" before the exchange) hero. Whether it's the entire army, part of the army, or a single unit doesn't depend on the mod, SoD, WoG, or any other version. And it doesn't need to. Before splitting up a stack, for example, it's desirable to have the focus on it, even if it's a single unit. So, the problem is, at the very least, not correctly formulated.
In versions 2.9x, there is an option to disable the second button that closes the exchange window (focusing on the right, "passive" hero). In 3.0 (I've only installed beta 11 so far, but I think it's the same in the others), it doesn't seem to be disabled, but there's a regular button with a checkmark (or Enter on the keyboard) that works like everywhere else, and there's another button with a hero's head and an arrow to the right for focusing on the right hero. If you don't need it, just don't touch it.