You do not need to equip the weapon once it's equipped.
Equip the axe and the knife remains as an option in the combat screen.
Once the axe is equipped, forget the Equipment menu, it's irrelevant (
until you gain the Shotgun from the City & need to replace the rifle).
Only the combat menu is relevant, from that point.
Look, here, in the screenshots I already provided. Please, actually look at them, it will help you to understand:
Equipment menu:
View attachment 930145
Axe equipped.
Combat menu:
View attachment 930146
Knife (Quick Attack
only, top option)
View attachment 930148
Axe (Heavy Attack
only, bottom option)
Both are equipped.
The axe is the Heavy weapon.
If you choose to use the Quick Attack you are not using the axe, you are using the knife & this will be why you did not kill the boar in one hit.
You
cannot do Heavy Attacks with the knife.
You
cannot do Quick Attacks with the axe.
Neither option exists in the game.
I hope you can get it from this, because I honestly don't know how to make this any more clear.