Data assist search does not grouping in the way yo are expecting.
instead of using multiple form elements for the country, use only one multiple select list to list the countries, in data assist search, set the country criteria to a keyword type.
for the Keyword OR separator, use a comma followed by a space:
", "
for the and separator, enter:
"and "
set the name of the multiple select list to end in "[]" this will pass an array of the selections. and add the following code at line 1 to convert the array to a comma seperated list:
<?php
if($_SERVER["REQUEST_METHOD"] == "POST") {
foreach($_POST as $key => $val) {
if(is_array($_POST[$key])) $_POST[$key] = implode(", ", $_POST[$key]);
}
}
?>