Actually, no it's not breaking adsense TOS. The TOS specifically allows multiple pub IDs on 3 conditions:
1) The respective owners of the pub-ID gives explicit permission to publish their ads (your users allows you to publish their ads on your site)
2) The owner of the publishing site gives explicit permission to allow ads of other pub-ids (you allow your users to publish their ads on your site)
3) Not more than 3 ad space is displayed on any single page.
Read the
letter from Google in the plugin's documentation for more info: