Post_per_page and several loops

Hello
In my theme, I have a column with posts from one category only but with TWO different lay-outs.

? The first 10 posts have a larger lay-out with excerpt and thumbnail.

<?php query_posts ('posts_per_page=10&cat=6&orderby=date'); ?>

<?php if (have_posts()) : ?> (bla bla bla)

LAY OUT

Works ok. I have my 10 posts.

? Just below, same column, ten more posts but with smaller lay out (just thumbnail and title)

Another loop starts with the OFFSET parameter, not to repeat the first 10 posts.

<?php query_posts('posts_per_page=10&cat=6&orderby=date&offset=10'); ?>

<?php if (have_posts()) : ?> (bla bla bla)

LAY OUT

Ok, it starts as I want on the 11th post.

BUT PROBLEM : the second loop doesn’t stop and finally displays ALL the remaining posts of the category.

The second “post_per_page” seems not to be taken into account.

Why is that ? What shall I do ?