California Code, Code of Civil Procedure - CCP § 427.10

(a) A plaintiff who in a complaint, alone or with coplaintiffs, alleges a cause of action against one or more defendants may unite with such cause any other causes which he has either alone or with any coplaintiffs against any of such defendants.

(b) Causes of action may be joined in a cross-complaint in accordance with Sections 428.10 and 428.30.