Try this:

[ol]
[li]Go to your Menu Manager and edit the "Home" link, in the Layout tab set the "Leading Articles" to the amount you want on the homepage and set the "Intro Articles" to 0.[/li]
[li]Paste this into the custom CSS area of the Template Manager: .leading-0 {border-bottom:1px dotted #383535!important;} [/li]
[/ol]

Making the page only display leading articles for the amount you want will ensure Joomla renders each article the same so the CSS will work.

http://blackforesthunting.com/index.php/en/aktualnosci

As you can see, there are two articles. I would like to separate them with for example line.

Please provide a development link to the site and I might be able to help.