ok resolved with
<?php
$c = 0;
$args = array( 'post_type' => 'custompost', 'posts_per_page' => -1 );
$loop = new WP_Query( $args );
while ( $loop->have_posts() ) : $loop->the_post();
if($c % 2 == 0) :
?>
//showpost
<?php
endif;
$c++;
endwhile; ?>
and of course
if($c % 2 != 0) :
in the other query!