This is the solution:
add_filter( 'myblogs_blog_actions', 'my_sites_visit_site_target_blank', 10, 2 );
function my_sites_visit_site_target_blank( $actions, $user_blog ) {
$to_replace = ">" . __( 'Visit' ) . '</a>';
$to_replace_with = " target="_blank" >" . __( 'Preview 1' ) . '</a> | <a href="'. esc_url( get_home_url( $user_blog->userblog_id ) ).'/customfolder/" target="_blank">Preview 2</a>';
return str_replace( $to_replace, $to_replace_with, $actions );
}