Cal. RTC § 18522

If an individual has filed a separate return for a taxable year for which a joint return could have been made by him or her and his or her spouse under Section 18521, and the time prescribed for filing the return for that taxable year has expired, that individual and his or her spouse may nevertheless make a joint return for that taxable year, provided a joint federal income tax return is made under the provisions of Section 6013(b) of the Internal Revenue Code. A joint return filed by the spouses in that case shall constitute the return of the spouses for that taxable year, and all payments, credits, refunds, or other repayments made or allowed with respect to the separate return of either spouse for that taxable year shall be taken into account in determining the extent to which the tax based upon the joint return has been paid.