You can do it easily with jQuery by using .eq() to add a specific class to each i element.
Example:
Add a class to your i element from PHP.
<?php wp_list_pages('link_before=<i class="retina_icon"></i>&link_after=<br >'); ?>
Then add another class to it with jQuery.
jQuery('.retina-icon:eq(0)').addClass("retina-icon-1");
jQuery('retina-icon:eq(1)').addClass("retina-icon-2");