When some of the several defendants in a suit are served with process in due time and others are not so served, the plaintiff may either dismiss as to those not so served and proceed against those who are, or he may take new process against those not served, or may obtain severance of the case as between those served and those not served, but no dismissal shall be allowed as to a principal obligor without also dismissing the parties secondarily liable except in cases provided by statute. No defendant against whom any suit may be so dismissed shall be thereby exonerated from any liability, but may at any time be proceeded against as if no such suit had been brought and no such dismissal ordered.