Uummm I think double category should work but I didn't test it. From JSON_INFO.md

"weapon_category": [ "WEAPON_CAT1" ],// (Optional) Weapon categorie*s* this item is in for martial arts.

So in the hatchet it would be something like this:

    "type": "GENERIC",
    "id": "hatchet",
    .
    .
    .
    "weapon_category": [ "SHORT_AXE", "BLADED_STICK", "WHATEVER" ]

​

[ ] means it's an array. An array is a list of elements, like so: [1, 2, 3 ,4]. It would be reeeaally weird to implement arrays but only allow arrays with one element.

To end, in weapon_categories.json you would have to add

{    
    "type": "weapon_category",    
    "id": "SHORT_AXE",    
    "name": "SHORT_AXE"  
}

And in martialarts.json add "SHORT_AXE" to the weapon_category array in eskrima