[minorgod]

If you know how to code and are just looking for a list of files to modify without specific instructions, here's what I modded to add a second billing and shipping address line to a 3.5.x cart:

ALTER TABLE `xcart_customers` ADD `b_address2` VARCHAR( 64 ) AFTER `b_address` ;
ALTER TABLE `xcart_customers` ADD `s_address2` VARCHAR( 64 ) AFTER `s_address` ;
ALTER TABLE `xcart_orders` ADD `b_address2` VARCHAR( 64 ) AFTER `b_address` ;
ALTER TABLE `xcart_orders` ADD `s_address2` VARCHAR( 64 ) AFTER `s_address` ;

INSERT INTO `xcart_languages` ( `code` , `descr` , `name` , `value` , `topic` ) VALUES ('US', 'Address', 'lbl_address2', 'Address 2', 'Labels');


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PHP files altered:
\admin\pay_membership_fees.php
\admin\pay_subscriptions.php
\include\func.php
\include\register.php

Template files altered
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\skin1\admin\main\register.tpl
\skin1\admin\main\user_delete_confirmation.tpl
\skin1\customer\main\customer_details_fax.tpl
\skin1\help\contactus.tpl
\skin1\images\help\contactus.tpl
\skin1\mail\help_contactus.tpl
\skin1\mail\order_invoice.tpl
\skin1\mail\profile_data.tpl
\skin1\mail\html\help_contactus.tpl
\skin1\mail\html\order_invoice.tpl
\skin1\mail\html\profile_data.tpl
\skin1\main\orders_export.tpl
\skin1\main\order_info.tpl
\skin1\main\register_billing_address.tpl
\skin1\main\register_shipping_address.tpl
Skipped: \skin1\modules\QuickBooks\orders_export_qb.tpl
\skin1\provider\main\register.tpl