B. Separate returns may be filed under subsection A of this section only if there is paid in full all of the following, at or before the time of filing such returns:

1. 1. All amounts previously assessed with respect to both spouses for such taxable year.
2. 2. All amounts shown as the tax by the spouses upon their joint return for such taxable year.
3. 3. Any amount determined, at the time of the filing of the separate returns, as a deficiency with respect to the spouses for such taxable year, if, prior to such filing, a notice of proposed deficiency under section 42-1108 has been mailed.

C. Separate returns may not be filed under subsection A of this section:

1. 1. After the expiration of four years from the last date prescribed by this title for filing the return for such taxable year, determined without regard to any extension of time granted for the filing of the joint return.
2. 2. After there has been mailed to the spouses, with respect to such taxable year, a notice of deficiency under section 42-1108, or if the spouses, as to such notice, appeal to the department under section 42-1251, or appeal to the state board under section 42-1253.
3. 3. After the spouses have commenced a suit in court for the recovery of any part of the tax paid for the taxable year with respect to the joint return.
4. 4. After the spouses have entered into a closing agreement under section 42-1113, with respect to such taxable year as to the tax payable by the spouse under their joint return.

F. Where the aggregate amount of the taxes shown by the spouses on their separate returns filed pursuant to subsection A of this section exceeds the tax shown on their joint return:

1. 1. If any of such excess is attributable to negligence or intentional disregard of rules and regulations of the department, but without intent to defraud, at the time of making the joint return, then five per cent of the total amount of such excess on each return shall be assessed, collected and paid as if it were a deficiency.
2. 2. If any part of such excess is attributable to fraud with intent to evade tax at the time of the making of the joint return, then fifty per cent of the total amount of such excess shall be so assessed, collected and paid, in lieu of the fifty per cent addition to the tax provided in section 42-1125.