Если заказ на одной странице, то в шаблоне /themes/default-bootstrap/order-opc-new-account.tpl скрыть блок со вторым адресом
<div style="display:none"><!-- begin -->
<div class="checkbox">
<label for="invoice_address">
<input type="checkbox" name="invoice_address" id="invoice_address"{if (isset($smarty.post.invoice_address) && $smarty.post.invoice_address) || (isset($guestInformations) && isset($guestInformations.invoice_address) && $guestInformations.invoice_address)} checked="checked"{/if} autocomplete="off"/>
</div>
<div id="opc_invoice_address" class="is_customer_param">
....
</div>
</div> <!-- end -->