Cal. Code Regs. tit. 10 § 2603.05
In the event of appearance of new legal counsel after the filing of the initial application, authentic written authority from applicant for the appearance or substitution, as the case may be, shall accompany the first paper filed by the new or substituted attorney at law or shall be presented at the new or substituted attorney's earliest appearance, whichever shall first occur. Whenever it appears that two or more different attorneys at law (exclusive of "House Counsel") or other attorneys, or firms of attorneys at law are filing or have filed papers or originated correspondence on behalf of an applicant, the Commissioner reserves the right to require a warrant of authority from any or all of such attorneys before recognizing or responding to such papers or correspondence.
Cal. Code Regs. Tit. 10, § 2603.05