Revenue and Taxation Code Section 18522 provides that where an individual has filed a separate return for a taxable year for which a joint return could have been made by him and his spouse under Revenue and Taxation Code Section 18521, and the time prescribed by law for filing the return for such taxable year has expired, such individual and his spouse may, under conditions hereinafter set forth, make a joint return for such taxable year. The joint return filed pursuant to Revenue and Taxation Code Section 18522 shall constitute the return of the husband and wife for such year, and all payments, credits, refunds, or other repayments, made or allowed with respect to the separate return of either spouse are to be taken into account in determining the extent to which the tax based on the joint return has been paid.

The fact that the taxpayer and his spouse are divorced or legally separated at any time after the close of the taxable year for which separate returns are filed shall not deprive them of their right to file a joint return for such taxable year pursuant to Revenue and Taxation Code Section 18522.