title: Scrapy CSV header row format with Multiple spiders and CSVItemExporter
body: <p>I am running four spiders and exporting the data into one csv file. However, when the second spider runs  it and gets data it creates the same row with column names. I tried to format the row header with FEED_EXPORT_FIELDS but that did not work.</p>
  
  <p>What I am looking to do is have 1 row header and populate all the data below it for each spider. The image shows the error and below I gave an example of what I am looking to accomplish.</p>
  
  <p>I did look at CsvItemExporter but am not clear how I would get the data from all four spiders and export the data. I have read over the documentation but still don't see how I would tie all this together.</p>
  
  <pre><code>TMP_FILE = os.path.join(os.path.dirname(sys.modules['products'].__file__), 'tmp/allproducts.csv')
  FIELDS = ['url', 'company', 'location', 'price', 'make', 'model', 'year', 'height']
  
  process = CrawlerProcess({
      'FEED_FORMAT': 'csv',
      'FEED_URI': TMP_FILE,
      'FEED_EXPORT_FIELDS': FIELDS,
  })
  process.crawl(Spider1)
  process.crawl(Spider2)
  process.start()
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/jwufk.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/jwufk.png" alt="enter image description here"></a></p>