Ok I get something.
1- You create a php file. A page Template. What it means it starts like this:
<?php
/*
Template Name: Chuck Norris
*/
?>
You can change the template's name, or let 'Chuck Norris', as you want.
Then you write some code, for a post which will be displayed first:
<?php rewind_posts(); ?>
<?php
$posts = get_posts('category=14');
foreach($posts as $post) :
setup_postdata($post);
?>
<?php the_content(); ?>
<?php endforeach; ?>
As I want to display my category 14, where I just have 1 post, I write 14 in the adequated place below.
Then, we go for the second post to display:
<?php
$posts = get_posts('category=15');
foreach($posts as $post) :
setup_postdata($post);
?>
<?php the_content(); ?>
<?php endforeach; ?>
The same operation, with category 15.
Like this you can get an initially EMPTY page, without its own, normal loop, but full of FOREIGN posts.
Then, playing with html/css you can build a very cool home page (you can set it as a homepage in the Ad panel), with a main article, and some complementary articles, all of them build as posts in the Post Editor.
Enough for what I wanted.
I'm not able to understand properly the 'rewind' function which is in the first post, and which theoretically helps to regain the possibility to display a second loop. I was not able to set at the same time a normal loop in there, so the call for a rewind function is maybe not necessary here, because the only posts I'm building comes from 'somewhere else'. More techie, depurated info at http://codex.wordpress.org/Template_Tags/get_posts#Parameters
I hope this helps somebody. My code can be full of horrors even in the shortest line, so be carefull.