Is it possible to have ads from multiple adsense accounts in the same/different pages of the same site?
It wouldn't be wise to place two different Adsense account ads on the same page. Aside from that, I am sure the Adsense team would have a problem with it. Better ask them to be safe.
It will be fine if each page has ads of just 1 publisher ID. You can take DP for example, it shows thousands of publishers ads but each page has all ads of same person.