Since Google has announced that AdSense will now show advertisements from other Ad networks; you are permitted to add multiple Google-certified ad networks to compete for display ad space on your site.
This will help publishers generate more cash, adding more competition into a site. Again, you’re only allowed to add other Ad Networks along with your Google Adsense Ad as long as it is Google certified.
So when you check out your Ad Review Center account, don’t be surprised to see a lot of ad networks queuing up on your display ad space. It’s up to you then to choose which of these ad networks you will allow to display their ads on your sites.