title: Scrapy: Running multiple spiders from the same python process via cmdLine fails
body: <p>Here's the code:</p>
  
  <pre><code>if __name__ == '__main__':
      cmdline.execute("scrapy crawl spider_a -L INFO".split())
      cmdline.execute("scrapy crawl spider_b -L INFO".split())
  </code></pre>
  
  <p>I intend to run multiple spiders from within the same main portal under a <code>scrapy</code> project but it turns out that only the first spider has run successfully, whereas the second one seems like being ignored. Any suggestions? </p>