Cal. LAB § 5303

There is but one cause of action for each injury coming within the provisions of this division. All claims brought for medical expense, disability payments, death benefits, burial expense, liens, or any other matter arising out of such injury may, in the discretion of the appeals board, be joined in the same proceeding at any time; provided, however, that no injury, whether specific or cumulative, shall, for any purpose whatsoever, merge into or form a part of another injury; nor shall any award based on a cumulative injury include disability caused by any specific injury or by any other cumulative injury causing or contributing to the existing disability, need for medical treatment or death.