A DigitalPoint Forums thread asks if you are allowed to place two different Google AdSense ads from two different publishers on the same page? For example, you and your friend run a blog together. Can you place your ad on the top left, while his ad is on the top right of the same page?
The answer appears to be no.
According to Google AdSense Online Standard Terms and Conditions, section two, paragraph "AdSense for Content," last line:
You shall not display any Ad Unit on a page that contains Ads associated with another Google AdSense customer (e.g., Your Web hosting company), unless authorized to do so by Google.
This does not mean it is out of the question. You simply need to get special person from Google to do so.
Forum discussion at DigitalPoint Forums.