A single declaration may be filed jointly by a husband and wife having the same income year. If a joint declaration is filed by a husband and wife and they do not file a joint return for the income year, the estimated tax paid under the joint declaration may be treated as the estimated tax of either the husband or wife or may be divided between them at the election of the taxpayers. If the taxpayers fail to agree upon a division or application of the estimated tax paid, then the commissioner shall divide the estimated tax paid between the husband and wife or apply the estimated tax paid to the tax due by either the husband or wife.