Answer in Medium article on Design Tokens - Approach B

style-dictionary-config-desktop.json

{
  "source": ["tokens_desktop/spacing.json", "tokens_button.json"],
  "platforms": {
    "desktop": {
      "transformGroup": "desktop",
      "buildPath": "build/desktop/",
      "files": [{
        "destination": "tokens.ext",
        "format": "my/format"
      }]
    }
  }
}

style-dictionary-config-mobile.json

{
  "source": ["tokens_mobile_spacing.json", "tokens_button.json"],
  "platforms": {
    "mobile": {
      "transformGroup": "mobile",
      "buildPath": "build/mobile/",
      "files": [{
        "destination": "tokens.ext",
        "format": "my/format"
      }]
    }
  }
}

tokens_button.json

{
    "button": {
        "text-icon-spacing": {
            "value": "{spacing.lg.value}",
            "type": "size"
        }
        ...
    }
}

tokens_desktop_spacing.json

{
    "spacing": {
        "sm": {
            "value": "4",
            "type": "size"
        },
        "md": {
            "value": "8",
            "type": "size"
        },
        "lg": {
            "value": "12",
            "type": "size"
        },
        "xlg": {
            "value": "16",
            "type": "size"
        }
    }
}

tokens_mobile_spacing.json

{
    "spacing": {
        "sm": {
            "value": "2",
            "type": "size"
        },
        "md": {
            "value": "4",
            "type": "size"
        },
        "lg": {
            "value": "8",
            "type": "size"
        },
        "xlg": {
            "value": "12",
            "type": "size"
        }
    }
}