Idea behind this feature is: To enable separation of address data for clients who might need to store "House number" or any other part of address data in a separate required field.

• You can enable this feature from System > Configuration > Sales > OneStepCheckout > Exclude fields > Separate address fields = YES
• Enabling this feature will add labels and required rules to each address street field.
• Address street fields will be rendered side by side (you can tweak the css to fit your needs).
• You will need to localize the street field labels to fit your needs.
• The separate address lines can be accessed from address object $addressObject->getStreet(1) and $addressObject->getStreet(2); in code level.
• You can enable up to 4 fields from System > Configuration > Customer > Customer configuration
• the system wide address display template that affects orders, e-mails , pdf's is defined in app/code/core/Mage/Customer/etc/config.xml file (her's a explaining tutorial: http://magentodevelopmentservices.com/magento-change-customer-address-format.html)

NB: This will only mimic the field separation on OneStepCheckout page and other templates (like customer registration or data editing forms) are not affected.