Declarations of estimated tax; joint declaration by husband and wife

A husband and wife may file a joint declaration, in which case the liability with respect to the estimated tax shall be joint and several. A joint declaration may not be made: (i) if either the husband or the wife is a nonresident of Virginia unless both are required to file an individual income tax return, (ii) if they are separated under a decree of divorce or of separate maintenance, or (iii) if they have different taxable years. If a joint declaration is made but a joint return is not made for the taxable year, the estimated tax for such year may be treated as the estimated tax of either the husband or the wife, or may be divided between them as they mutually agree.