Why waste time doing that? 3 ad units are enough! You can't make more money by putting many ads on one page, it depends on where you put your ads and how you customize them.
I've said many times here that it is allowed and I still stand by my advised. However, it has never been fully clarified by Google (just like many other things). I can confirm to you one case that I know for sure where this is is allowed but on that case the publishers had to email Google directly. So just like having two adsense accounts are only permitted on case by case basis, I'd say you should email Google directly and show them your site.
yes you can do it but only 3 ads allow on each page but there is no proplem to put 2 adsense accounts on one site
You can have more than one publisher ads at the same time in a single page. You can read this post Share you Ad Space. Google has confirmed this kind of model. Refer this thread also http://forums.digitalpoint.com/showthread.php?t=957721
You can only earn one Adsense account, it is against the rules to own more than one. Be on the safe side, don't register for another Adsense account, you'll just get your primary and you're secondry one terminated. Good luck.
You can show out as many adsense IDs, as you like to in your site, but you cannot put two different adsense ids on the same page. The adsense accounts will belong to different people off course as one person can only have one adsense account. However, you can do revenue sharing like DP in your site.