2In estimating and certifying the expenditure on the transferred services by a county or county borough council for the purposes of the foregoing rule in any case where the area of a highway authority or poor law authority is not wholly comprised in one county or county borough, the expenditure of the authority shall be apportioned between the several counties and county boroughs into which the area extends, and the amount apportioned to any such county or county borough shall be deemed to be expenditure by the council of that county or county borough on the transferred services.