Adsense: Sharing Your Ad Space

A number of publishers have asked us if it's acceptable to place multiple publishers' ad code on the same page or site. Often they'll run into this issue when two AdSense publishers manage a site together and both want to display their ads, or if a site owner hosts content and ads from other publishers on the same page as their own.
The answer is that yes, we do allow this. But before you place your ad code on the same page or site as another publisher, here are a few important points to keep in mind:

The maximum number of products per page will remain the same.
If you don't own the site, you'll need to receive permission from the site owner to display your ads on their pages.
We're unable to divide earnings between accounts, so any revenue generated will be credited to the account associated with the ad unit that was clicked or viewed.
Any shared site that displays Google ads must also comply with our program policies.
You're responsible for the content of any page where your ad code appears. Before placing your ad code on another publisher's site, we recommend reviewing the site for any policy violations.

And one final tip before giving your code to another publisher -- try using our Allowed Sites feature to monitor where your ads are appearing and keep your account in good standing.

source: AdSense Publisher Support