If you want don’t want to show content if the current template is template-custom.php
or template-custom2.php
you can use:
if (!is_page_template('template-custom.php') && !is_page_template('template-custom2.php')) {
<!-- show some content when you AREN NOT in template-custom.php NOR template-custom2.php -->
}
or
if (is_page_template('template-custom.php') || is_page_template('template-custom2.php')) {
<!-- show some content when you ARE in template-custom.php OR template-custom2.php -->
}