Yes, according to AdSense Policy you can use more than AdSense Account on one Website. Publisher now add two AdSense account’s ad code in one website. Sometimes this facility for business perspective.
If you have Two different AdSense Account you can add both account ad code in same one website, but you should maintain AdSense police that you can only add three AdSense ad code in one website.
Sometimes you can see more than 3 AdSense ad zones in a single webpage. Many bloggers and website developers always wants to know how more than 3 ad zone are displaying in one webpage. This is happen by AdSense two account policy for one website.
Dual accounts ads policy definitely increase revenue of the website because 6 ads zone must grow the chance of clicking on ads.