Where the action is against two or more defendants, and one or more shall have been served, but not all of them, the plaintiff may proceed as follows: (1) If the action be against defendants jointly indebted upon contract, he may proceed against the defendant served, unless the court otherwise direct; (2) if the action be against defendants severally liable, he may, without prejudice to his rights against those not served, proceed against the defendants served in the same manner as if they were the only defendants.