personally, I would show this in one list and float the <li> elements left. Possibly add an odd an even class so that I could add a clear left on the odd ones. This way on a really narrow screen they would still be listed in the correct order.
As for your question, something like this
if (have_rows('repeater')) {
$column_1 = array();
$column_2 = array();
$count = 0;
while (have_rows('repeater')) {
the_row();
if ($count % 2 == 0) {
$column_1[] = get_sub_field('sub_field');
} else {
$column_2[] = get_sub_field('sub_field');
}
$count++;
}
}
// now create your ul elements and loop through each array
