https://github.com/swagger-api/swagger-ui/issues/4485

4485-book.json

{
  "post": {
    "description": "Book",
    "operationId": "buy",
    "summary": "Buy a book",
    "tags": [
      "Book"
    ],
    "consumes": [
      "application/json",
      "application/xml"
    ],
    "parameters": [
      {
        "name": "requestBody",
        "in": "body",
        "description": "Buy a Book",
        "required": true
      }
    ]
  }
}

4485-combined.json

{
  "swagger": "2.0",
  "paths": {
    "/v1/book": {
      "post": {
        "description": "Book",
        "operationId": "buy",
        "summary": "Buy a book",
        "tags": [
          "Book"
        ],
        "consumes": [
          "application/json",
          "application/xml"
        ],
        "parameters": [
          {
            "name": "requestBody",
            "in": "body",
            "description": "Buy a Book",
            "required": true
          }
        ]
      }
    }
  }
}

4485-main.json

{
  "swagger": "2.0",
  "paths": {
    "/v1/book": {
       "$ref": "./4485-book.json"
   },
  }
}