Custom query problem

I have a small problem with a custom query. here’s the code I have so far:

<?php $recentPosts = new WP_Query();
$recentPosts->query('cat=1,19&showposts=6');
while ($recentPosts->have_posts()) : $recentPosts->the_post();
$no_duplicate_recentPosts = $post->ID; ?>
<!-- do stuff -->
<?php endwhile; ?>

Now my question, how I have to adjust the

$no_duplicate_recentPosts = $post->ID;

so the constant includes all 6 posts from the query above?
Later I basically want to display the next 4 posts from the same 2 categories in a separate loop, obviously without those first 6 posts with this code:

<?php $recentPosts2 = new WP_Query();
$recentPosts2->query('cat=1,19&showposts=10');
while ($recentPosts2->have_posts()) : $recentPosts2->the_post();
if($post->ID == $no_duplicate_recentPosts) continue; update_post_caches($posts); ?>
<!-- do stuff -->
<?php endwhile; ?>

Thanks a lot…