@mikker's config

Mapping double-cmd to F12:

Use Karabiner-Elements.

Add Complex modification:

{
    "description": "Akimbo cmd to F12",
    "manipulators": [
        {
            "from": {
                "simultaneous": [
                    { "key_code": "left_command" },
                    { "key_code": "right_command" }
                ],
                "simultaneous_options": {
                    "to_after_key_up": [
                        {
                            "key_code": "f12",
                            "modifiers": ["fn"]
                        }
                    ]
                }
            },
            "to": [{ "key_code": "vk_none" }],
            "type": "basic"
        }
    ]
}