if you dont try to merge your posts into one list you could try this:
<ul class="program-list">
<?php query_posts('cat=2'); ?>
<?php if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>
<?php
$repeater = get_field('program_repeater');
foreach( $repeater as $key => $row ) {
$thedate = $row['program_repeater_date'];
$column_id[ $key ] = strtotime($thedate);
}
array_multisort( $column_id, SORT_ASC, $repeater );
foreach( $repeater as $row ){
$dateDayNumber = date_i18n("d", strtotime(get_sub_field('program_repeater_date')));
$dateMonthNumber = date_i18n("m", strtotime(get_sub_field('program_repeater_date')));
$dateDayOrg = date_i18n("l", strtotime(get_sub_field('program_repeater_date')));
$dateDay = substr($dateDayOrg, 0, 2);
$dateTime = get_sub_field('program_repeater_time');
$specialEvent = get_sub_field('program_repeater_special');
$mainTitle = get_the_title();
$subTitle = get_field('sub_text');
$rowSeatsId = get_sub_field('program_repeater_rowseats_id');
?>
<li>
<div class="specialEvent"><?php if( $specialEvent ): ?><?php echo $specialEvent; ?><?php else: ?> <?php endif; ?></div>
<div class="dateDayNumber"><?php echo $dateDayNumber; ?></div>
<div class="dateMonthNumber"><?php echo $dateMonthNumber; ?></div>
<div class="dateDay"><?php echo $dateDay; ?></div>
<div class="v-line-zick-zack"></div>
<div class="dateTime"><?php echo $dateTime; ?></div>
<a href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>">
<div class="mainTitle"><?php echo $mainTitle; ?></div>
<div class="subTitle"><?php echo $subTitle; ?></div>
</a>
<a href="<?php the_permalink(); ?><?php echo "?id=" . $rowSeatsId; ?>" title="<?php the_title_attribute(); ?>">
<div class="ticket-link">Tickets kaufen</div>
</a>
<div class="clear"></div>
</li>
<?php } ?>
<?php endwhile; endif; ?>
<?php wp_reset_query(); ?>
</ul>
If you try to merge them => you need to build a array first, order it, and echo it after that.
try to adapt a solution like this or this
