I would use the closest layout to your needs, like “Separate Lines” layout, in the plugin settings:
WooCommerce > Simple Shipping Labels > Recipient tab > Details Layout
then adjust the code in /admin/admin.php file, within the labels page generation function:
function generate_shipping_labels_page_html( $orders ) {...}
above the switch statement:
switch ( $this->settings['recipient_details_layout'] ) {...}
you can split the HTML of the address 1 and 2 line:
<div class="detail-container address"><div contenteditable="true" class="detail<?php echo $this->rtl_css_class($order_address_1) ?>"><?php echo esc_html( $order_address_1 ) . ( (strlen($order_address_2) == 0) ? "" : " " . esc_html( $order_address_2 ) ) ?></div></div>
Just duplicate this line and remove the address 1 or 2 from one another.
You can do the same with the layout preview (in plugin settings) in the layouts previews array:
public $recipient_details_layout_options = array(...);