I think what might happen is if you have more adverts beinig displayed you will be more likely to also have lower paying adverts displayed. And if these lower paying adverts are also displayed, and being clicked, You will earn less through those clicks.
But then, If those adverts weren't there who is to say your visitor would have clicked the other adverts instead?
If you have one adblock google will be liekly to show the higest paying adverts in that block. But the more adblocks that are displayed the less high paying adverts there are, so you will be more likely to have the adverts paying less the more adverts there are on the page..
So having more adblocks won't mean you automatically earn less, But it will mean there will be more of a chance that there will be lower paying adverts displayed on the page.
Actually, you should have the first ad block on the area where you expect maximum clicks. Google gives max. paying ads for the first ad block. You can have a second ad block somewhere below it(rather after the best area, in HTML code), but in my exprience, the best option is to have just one (max 2) ad on the area where you get max clicks. That way your vistors too will be happy, not to see too many ads.
