I read that getting double positions on Google (ex position 7 and 8) would double the amount of traffic comming from Google. How do I do this? Is it just to do the exact same onsite SEO with the exact same keywords for 2 HTML pages, or is it more to it?
Yep, you need to optimize 2 different web pages for the same keyword or keyword phrase. It's not an easy task as it's hard to build the needed backlinks. Most websites won't link the same anchor text to the same website twice for 2 different pages.
Also, to get the subpage to rank you ONLY need to get on the same page as the homepage and it will show up right underneath it...so lets say you rank #1 for cars and your subpage about sports cars ranks #11...you only need to move up 1 spot and you will actually end up moving up 10 spots to #2 right below your #1 listing.
As other said, you can get indent listing by optimizing 2 pages for the same keywords. I will try to explain in simple way (hopefully you can understand) how you can obtain double listing (indent listing). The easiest way is when you have already reached a decent ranking (mainly on the first page of the SERPs) with one page for that same keyword, lets call this page A. So you have this page A already optimized and ranking for your desired keywords, and now you create another page treating the same argument, more in depth if possible (but make sure it is informative and not just spam). Create a similar title for the page B (the one which we want to get indent listed) as the page A (but not identical, remember, needs o be informative and not to treat the same exact argument as page A but rather something related to page A or a more in-depth explanation). Also, the keyword density and prominence in the title of page B needs to be lower than the one of page A. Once you have created this page B and optimized it (title, meta description, heading element, keywords in content etc) interlink the page A with the page B by using your desired keywords (and vice versa). If you can, interlink the homepage of your site with these pages as well. With the help of some (not too much) inbound links from external sites by using your desired anchor text in the link you should obtain indent listing for your site. To make things more clear, don't use always the same exact keywords as anchor text, lets say page A's title is "Car Reviews" and page B's title is "Hot Car Reviews", you can concentrate most of the keyword in anchor text as "Car Reviews" for page A while just a few for page B, but mainly page B should get inbound (from external and internal) links with its proper anchor text (hot car reviews).