Call register_sidebar() for each sidebar you want to create.
# Define the 'main-sidebar'.
add_action( 'widgets_init', 'my_register_sidebars' );
function my_register_sidebars(){
register_sidebar(
array(
'id' => 'main-sidebar',
'name' => __( 'Main Sidebar' ),
'description' => __( 'Widgets in this area will be shown on all posts and pages.' ),
'before_widget' => '<aside id="%1$s" class="widget %2$s">',
'after_widget' => '</aside>',
'before_title' => '<h3>',
'after_title' => '</h3>'
)
);
/* Repeat register_sidebar() code here for each additional sidebar. */
}