If any county officer performs municipal duties imposed by a charter framed under Section 8 of Article XI of the California Constitution, the compensation and expense of the officer may be apportioned by the board of supervisors in proportion to the duties performed as a county officer under the general laws and performed as a municipal officer under the charter provision. The compensation determined to be for the performance of municipal duties shall be paid from municipal funds and the compensation determined to be for county duties shall be paid from county funds.