gistfile1.php

<?php 
	// Add a post signature for multiple authors.
	// All signature divs will have a class of .signature for common styling
	// then .signature-cat as the default and .signature-tracey for the guest
	// as class modifiers (different background image, or background position
	// if using sprites etc.)
	
	// If it's not a single entry of any post type, return.
	if ( is_single() ) { 
		return;
	}

	// Defaults for main author.
	$class_suffix = 'tracey';
	$name = 'Tracey';

	// Cat is the guest.
	$cat_user_id = 2; // Change this to Cat's user ID.
	if ( get_the_author_meta( 'user_id' ) == $cat_user_id ) {
		$class_suffix = 'cat';
		$name = 'Cat';
	}

	// Name added for better accessibility - you can use the .signature class to
	// implement your favourite image replacement technique.
?>
<p class="signature <?php echo sanitize_class_html( 'signature-' . $class_suffix ); ?>"><?php echo esc_html( $name ); ?></p>

Be sure to change the guest's user ID!