No, this is explicitly forbidden by the Google AdSense Terms and Conditions and Programme Policies. If you really want to do this, then use some PHP or Javascript code to randomly select between the publishers. Search forum for "ad rotate code". Cryo.
I have read on other threads on DP that its ok if there are 2 different publishers' ad units of adsense until they are following the rule of no more than 3 ad units and 1 link unit placing on single page.
The definite answer is "no". Even though a max of 3 ad units are allowed, 2 pub ids on the same page is definitely a NO.
So, on here, would that mean that both the ad at the top of the forum, and in the forum conent, would have to change to the same publisher?