If you pasted the exact second block you’ll get errors because you are breaking up the closing and opening PHP tags <?php and ?>. Try this instead, or have a look at this for a better understanding, notice the difference …
if ( is_page( 'about' ) || '29' == $post->post_parent ) {
//Start of the custom code...
while(has_sub_field('site_slideshows','option')):
?> <!-- Closing the PHP tag -->
<div class="slideshow-slide" style="background-image:url(<?php the_sub_field('site_slideshows_slide_image'); ?>)">
</div>
<?php //Opening the PHP tag again
endwhile;
} elseif ( is_page( 'contact' ) || '30' == $post->post_parent ) {
//Start of the custom code...
while(has_sub_field('site_slideshows','option')):
?> <!-- Closing the PHP tag -->
<div class="slideshow-slide" style="background-image:url(<?php the_sub_field('site_slideshows_slide_image'); ?>)">
</div>
<?php //Opening the PHP tag again
endwhile;
}