Switching Themes using conditionals

Hi WP friends,

I´m trying to make a simple plugin to help me to show up a diferent theme for by blog posts then the rest of the website.

After search a lot, I found some filters used to do it. But I can´t get it to work well… this is the code of the plugin:

add_filter('template', 'the_parent_theme_directory');
add_filter('option_template', 'the_parent_theme_directory');
add_filter('option_stylesheet', 'the_child_theme_directory');

function the_child_theme_directory() {

	if (is_single()) {
		return 'child_theme1';
	} else {
		return 'child_theme2';
	}
}

function the_parent_theme_directory() {

	return 'parent_theme';
}

The problem is that when “is_single()” it just run the style.css and don´t run the functions.php of the child_theme1. But when is “else”, it runs everything, css, functions, templates…

What´s wrong? Any help?
Thanks!!

[No bumping, thank you.]