这里有一个（未经测试的）想法，我们将sidebar-inject 之前的侧栏sidebar-target 借助于侧栏dynamic_sidebar_before 挂钩：

add_action( \'dynamic_sidebar_before\', \'wpse_inject_sidebar\', 10, 2 );

function wpse_inject_sidebar( $index, $has_widgets )
{   
    // Only target front-end
    if( is_admin() )
        return;

    // Only target \'sidebar-target\'
    if( \'sidebar-target\' !== $index )
        return;

    // Make sure \'sidebar-inject\' is active
    if( ! is_active_sidebar( \'sidebar-inject\' ) ) 
        return;

    // Avoid recursive loop
    remove_action( \'dynamic_sidebar_before\', \'wpse_inject_sidebar\', 10 );

    // Inject  \'sidebar-inject\' 
    dynamic_sidebar( \'sidebar-inject\' );

    // Re-hook it again
    add_action( \'dynamic_sidebar_before\', \'wpse_inject_sidebar\', 10, 2 );       
}

但只针对\'sidebar-target\' 在这里，确保它与\'sidebar-inject\', 我们实际上避免了递归循环。