Use specific template when multiple post types defined via pre_get_posts hook.

functions.php

<?php
/**
 * Modify Main Query
 *
 * Add multiple post types to main query.
 *
 * @param  object $query main query object
 * @return no return value necessary.
 */
function _s_post_type_1_archive( $query ) {

    if ( ! is_post_type_archive( 'post_type_1' ) || is_admin() || ! $query->is_main_query() )
        return;

    $query->set( 'post_type', array( 'post_type_1', 'post_type_2' ) );

}
add_action( 'pre_get_posts', '_s_post_type_1_archive' );





/**
 * Force Use of Template
 * 
 * It's not clear to WordPress which template to use when querying multiple
 * post types. This check forces the use of a specific template
 *
 * @return void
 */
function _s_redirect_template( $template ) {

    // cast as an array when there's only a single post type
    $post_types = (array) get_query_var( 'post_type_1' );

    // check for both resource and faq as post types
    if ( in_array( 'post_type_1', $post_types ) && in_array( 'post_type_2', $post_types ) ) :

        $new_template = locate_template( array( 'archive-post_type_1.php' ) );

        if ( '' != $new_template )
            return $new_template;

    endif;

    return $template;

}
add_action( 'template_include', '_s_redirect_template' );

This is really old, but it's just helped me out tremendously, and I wanted to point out one tiny error:

$post_types = (array) get_query_var( 'post_type_1' );

should be:

$post_types = (array) get_query_var( 'post_type' );

Thanks for this.