Filtering posts based on two categories

Hi,

I have a question. Currently I am using this code, which was hammered out for me a while ago by MichaelH.

<?php
$cat=get_cat_ID('Exhibitors');
$catposts=get_posts('numberposts=999&order=ASC&cat='.$cat);
if ($catposts) {
  foreach($catposts as $post) {
    setup_postdata($post); ?>

It works awesomely, however, I am trying to mod it so that it pulls posts filtered by two categories, not just one.

So for example: a post will only show up on this page if it is in both, featured guests and kids. Currently i’ve been messing around with some syntax, but i get errors. Its probably a small fix, i’m just really new to PHP.

Any suggestions would be really helpful.

Thank you.