You need three cabin crew in a medical emergency that involves doing CPR.
One to do the compressions and one to hold the airway open and administer the breaths of air. (Ideally another to administer the shocks) the third one needs to liaise with the Flight Crew and keep them informed.
Doctors and nurses are great for prescribing, diagnosing and administering medication but believe it or not, most cabin crew are more used to practicing CPR and using a defibrillator than a lot of Doctors are.