$layouts = get_field('field_name', false, false);
$count = 0;
while (have_rows('flex_field')) {
the_row();
// normal code for displaying your layout here
if ($count == count($layouts)) {
// end of layouts reached
// do end of loop code
}
if (isset($layouts[$count+1])) {
if ($layout[$count+1] == 'Layout B') {
// next layout is layout be
// do that code
}
}
// increment count
$count++;
} // end while have rows