Go to your Google webmasters' account and choose site links option and Google shows extra links when it thinks it is fine
I know that is what you meant. And it is not in your controls but you can opt for that from your webmaster tools account. Check here, there are more, not just 2 links.
This will come up naturally especially if you have highly focused website or content. For example, it will tend to show more if: Say you have a guitar playing website, all of your pages are about guitar tutorials.. so it is a very relevant website. When the site will be shown in search result, Google shows a supplement pages that additional pages that will support the query in addition to the first one. They do this to maximize user experience. But if you have wide niche website, say not only your site about guitar playing but also about piano lessons, flute lessons.. etc. .. Google cannot pin point supplement or other related pages , so in this case, it will lower your chances to have the other shown in the search result.
google shows additional sites (plus sign) while other sites from your domain are also relevant for the keyword. in my experience deeplinks to your site and google sitelinks are the keys to get such a result,
I think it is depend upon the content of 2nd page or the rest of the pages, if it matches with the first one or densely keyword reached contents.
I think that come up by its own though google... thers nothing you can do except quality and focused content
It is not up to you. Google will give you double links whne it is relevant. For example, if you Google my name, my main site comes up along with my profile page from the same domain.
It is not in your hands, the search engine shows the deep links when it thinks that your secondary page has content that supplements the first page. Keywords, content play a crucial role in this.
We can't influence those links, google automatically shows 2-3, or more links from the same site (like in pictures above) when he thinks it's helpful for the user.