def parse_page1(self, response):
    item = MyItem()
    item['main_url'] = response.url
    request = Request("http://www.example.com/some_page.html",
                      callback=self.parse_page2)
    request.meta['item'] = item
    return request

def parse_page2(self, response):
    item = response.meta['item']
    item['other_url'] = response.url
    return item

scrapy.Request("http://www.example.com.html",callback=parse_page1)`

scrapy.Request("http://www.example.com/some_page.html",callback=parse_page2)

item['main_url'] = response.url # send "http://www.example.com.html" to item
request = scrapy.Request("http://www.example.com/some_page.html",
                         callback=self.parse_page2)
request.meta['item'] = item  # store item in request.meta

item = response.meta['item'] #response.meta is equal to request.meta,so  the item in request.meta of page1 is send to request.meta of page2,and here item['main_url'] ="http://www.example.com.html".

item['other_url'] = response.url # response.url ="http://www.example.com/some_page.html"

return item #finally,we get the item recordind  urls of page1 and page2.