Even if the second pair of eyes is adequately trained and has adequate experience and the issues of CRM between two totally differ backgrounds have been addressed, a member of the medical crew can never be guaranteed to be effective. The medical crew member will be aware of the incident, may well have been briefed about the pathology and will invariably be reviewing in his mind the protocol for the pathology, the specific risks, the possible mechanism of injury etc etc.
Behavioural psychologists have researched this scenario ad nauseam. Concentration on the flying task will always be degraded, and more worryingly the individual often has no insight into this. That is why we operate a Chinese wall so the pilots are unaware of the medical situation as far as possible.