location ~* ^/(css|js)/.+\.(css|js)$ {
        #checking if referer is from app1
        if ($http_referer ~ "^.*/app1"){
            return 417;
        }

        #checking if referer is from app2
        if ($http_referer ~ "^.*/app2"){
            return 418;
        }
    }
    error_page   417  /app1$request_uri;
    error_page   418  /app2$request_uri;


    location /app1 {
         proxy_pass  http://app1.com;
    }

    location /app2 {
        proxy_pass http://app2.com;
    }