If an estate or trust is a resident of this State and also a resident of another state, it shall, notwithstanding the limitations contained in Sections 18001 and 18002, be allowed a credit against the taxes imposed by this part for net income taxes imposed by and paid to the other state, subject to the following conditions:
(a)
Credit shall be allowed only for such proportion of the taxes paid to the other state as the income taxable under this part and also subject to tax in the other state bears to the entire income upon which the taxes paid to the other state are imposed.
(b)
The credit shall not exceed such proportion of the tax payable under this part as the income subject to tax in the other state and also taxable under this part bears to the entire income taxable under this part.