OK So I figured out how to add the second filter using a second trigger by referring another project using a similar technique and manually updating the trigger code as follows.
if (isset($_GET['county']) || isset($_GET['agent'])) {
.......
}
What I need now is for the page to show all by default or once a filter has been selected to be able to revert back to 'view all'
I have a button on the page such as:
<a href="right-to-manage-success-t.php" role="button"> View All </a>
But I'm not sure how I can add a parameter to this and then add a third filter to the search behaviour for this to work.