This is (and has been for years) specifically excluded per the Companion Pass T&Cs. It hasn't worked.
CPs cannot be chained.
From the Companion Pass Rules:
The Member must first book and purchase his/her flight in cash or points through Southwest Airlines and then book the Companion's flight through Southwest Airlines by choosing the Rapid Rewards Companion Pass booking option on Southwest.com, or if calling our toll-free reservations number at 1-800-248-4377, notifying the Customer Representative that a Rapid Rewards Companion Pass will be used. .....
... An individual may not travel as both a Member and a Companion on the same flight. ....
To add a CP reservation, the CP Holder's reservation must be booked with Cash or Points (or, though not stated, a Standard Award)...so, a person who is already a companion hasn't met this criteria. Further, the Rules specifically state that one cannot fly as both a CP holder and CP companion on the same flight.