How can I split my query result in 2 arrays?

There’s no need to split the array, you can just close the first div once you’ve counted up to four elements of the array.

$args = array(
    'post_type' => 'event',
    'meta_key' => 'date',
    'orderby' => 'meta_value_num',
    'order' => 'ASC',
    'posts_per_page' => 8,
    'meta_query' => array(
        'key'       => 'date',
        'value'     => date('Y-m-d',strtotime("today")),
        'compare'   => '>=',
        'type'      => 'DATE'
    ),
);

$events = new WP_Query( $args ); 
$count  = 1;

if ( $events->have_posts() ) : ?>
    <div class="column-1">
    <?php while ( $events->have_posts() ) : $events->the_post();
        // YOUR CONTENT
if ( $count == 4 ) { ?>
    </div>
    <div class="column-2">
<?php }

        $count++;
    endwhile;
    wp_reset_postdata();
endif;
?>
</div>