I worked it out:
<ul class="col-md-6">
<?php $i = 0; $j = count( get_field('repeater_field') );?>
<?php if( have_rows('repeater_field') ): while ( have_rows('repeater_field') ) : the_row(); ?>
<?php the_sub_field('sub_field'); ?>
<?php if ( ( $i + 1 ) == ceil($j / 2) ) echo '</ul><ul class="col-md-6">'; ?>
<?php $i++; endwhile; ?>
<?php endif; ?>
</ul>