The board of supervisors at the direction of governing board of the district may divide the principal amount of bonds authorized at any election into two or more series and may fix different dates for the bonds of each series, in which event the maximum maturity date of the bonds shall be calculated from the date of each series respectively. When the issuance of bonds shall have been authorized pursuant to two or more propositions submitted at the same or different elections, all or any part of the bonds not theretofore issued may be combined and issued and sold as one or more series.