You need to create a loop and use a counter to see what the current item is. This is where the php % (modulus) operator comes in handy http://php.net/manual/en/language.operators.arithmetic.php
<?php
if (have_rows('repeater')) {
$count = 0;
?><ul><?php
while(have_rows('repeater')) {
the_row()
if ($count > 0 && (($count % 2) == 0)) {
// skip the first one
// add list end/start whenever
// count is divisible by 2
?></ul><ul><?php
} // end if even
?><li><?php the_sub_field('sub_field_1'); ?></li>
$count++
} // end while have rows
</ul><?php
} // end if have rows
?>
