Different templates for same content, landing page from different URLs (generic, simple)

I don’t know what your definition of “simple” is, so not sure this qualifies. It’s not really a simple task to do this dynamically.

First, we register a new query var to pass the alternate identifier:

function wpd_portfolios_query_var($query_vars){
    $query_vars[] = 'portfolio_key';
    return $query_vars;
}
add_filter('query_vars', 'wpd_portfolios_query_var');

Next, we add a rewrite rule to handle incoming requests, and set the query var accordingly:

function wpd_portfolios_rewrite(){
    add_rewrite_rule(
        'portfolios/([^/]+)/?$',
        'index.php?post_type=portfolio&portfolio_key=$matches[1]',
        'top'
    );
}
add_action( 'init', 'wpd_portfolios_rewrite' );

Then we hook parse_query, check if our query var is set, and convert these requests to singular post requests. Otherwise WordPress will default to setting these requests to is_home.

function wpd_portfolios_parse( $query ){
    if( isset( $query->query_vars['portfolio_key'] ) ){
        $query->is_home = 0;
        $query->is_single = 1;
        $query->is_singular = 1;    
    }
}
add_action( 'parse_query', 'wpd_portfolios_parse' );

Next we hook pre_get_posts to set a meta query for the portfolio post with the desired meta value.

function wpd_portfolios_query( $query ){
    if( isset( $query->query_vars['portfolio_key'] ) ){
        $query->set( 'posts_per_page', 1 );
        $meta_query = array(
            array(
                'key' => 'portfolio_key',
                'value' => $query->query_vars['portfolio_key']
            )
        );
        $query->set( 'meta_query', $meta_query );
    }
}
add_action( 'pre_get_posts', 'wpd_portfolios_query' );

And lastly, we filter single_template to load the special template for these requests:

function wpd_portfolios_template( $template="" ){
    global $wp_query;
    if( isset( $wp_query->query_vars['portfolio_key'] ) ){
        $template = locate_template( 'portfolio-special.php', false );
    }
    return $template;
}
add_filter( 'single_template', 'wpd_portfolios_template' );