It isn't what Google pays, it's what advertisers bid.
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.
This is speculative, but I think smart pricing takes CTR into account. If you have a "low" CTR, the smart pricing discount is larger. Whether it's also possible to have a "too high" CTR, I don't know. No personal experience.
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.
Well I personally do not believe that ECPM and the number of impressions have NO effect on your earnings. Something called smart pricing does exist and we do not clearly know what it's algorithm does.
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]