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And don't obsess about EPC--effective CPM and total revenues are the numbers that matter.
If you add a second ad unit, you may earn more or less, depending on the ads available and user behavior. The only way to know how a second ad unit will perform on your site is to test--something that's extremely easy to do with AdSense.
To me, that would help to explain why, when I've removed low-CTR pages or channels, my EPC goes up. All pages with AdSense code have only one adblock, by the way.
What I have seen however is showing one ad on a page more profitable. The reason is very simple to understand. Google will show the most high performing ads on your first block. If there is a second block it kind reduces the probability of getting that click on the first block. However, with the addition of the CPM ads, I think things have become a bit more complicated. I think if you have CPM ads on your website, it's best to have two ad blocks and maybe an Adlink [if it performs well for you]