If the income of two or more taxpayers is derived solely from sources within this state and their business activities are such that if conducted within and without this state a combined report would be required to determine their business income derived from sources within this state, then such taxpayers shall be allowed to determine their business income in accordance with Section 25101.
Source:
Section 25101.15, https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=RTC§ionNum=25101.15..