You can place adsense codes from different adsense account in your single webpage. Might be you are running a website with your friend and you might be searching for Adsense Terms of service (TOS) on it.
Google states “Publishers are permitted to place ad code from different AdSense accounts on the same page as long as the owner of the website has given permission for ads to be placed on their site, and the webpage complies with our program policies.
A publisher will only be credited for clicks and impressions on the ad units associated with their account. Additionally, publishers are responsible for monitoring each webpage upon which their ad code appears to ensure compliance with our policies.”
You can put your code on any sites you wish – There are no restrictions other than the site must meet the Adsense TOS