You can use a plugin like below:
http://wordpress.org/extend/plugins/author-advertising-plugin/
http://wordpress.org/extend/plugins/author-advertising-pro/
Show your ads on your posts, his ads on his posts.
For common areas split ads 50:50.
If you are only-2 people then you can track traffic share among you from Google Analytics by configuring custom variable in http://wordpress.org/extend/plugins/google-analytics-for-wordpress/