Install the phpld deeplinks mod before applying the modification below.

To validate the additional deeplinks you will have to add the following:

First in submit.php
AFTER:

SmartyValidate :: register_validator('v_URL',
'URL','isURL', false, false, 'trim', 'submit_link');

ADD:

SmartyValidate :: register_validator('v_URL1',
 'URL1','isURL',true,false,'trim','submit_link');
SmartyValidate :: register_validator('v_URL1_ONLINE',
 'URL1','isURLOnline',true,false,null,'submit_link');

FINALLY in submit.tpl
AFTER:

<input type="text" name="URL1"
value="{$URL1|escape|trim}" size="40"
 maxlength="255" class="text"/>

ADD:

{validate form="submit_link" id="v_URL1"
 message=$smarty.capture.invalid_url}
{validate form="submit_link" id="v_URL1_ONLINE"
 message=$smarty.capture.url_not_online}

This will allow to validate the URL1 field when it is not empty. You should do the same for all URLs that you want to validate (straightforward).