The rights of the several parties, plaintiffs as well as defendants, may be put to issue, tried and determined by such action; and when a sale of the premises is necessary, the title shall be ascertained by proof to the satisfaction of the court, before the judgment of sale shall be made; and where service of the complaint has been made by publication, like proof shall be required of the right of the absent or unknown parties before such judgment is rendered; except that where there are several unknown persons having an interest in the property, their rights may be considered together in the action, and not as between themselves.