A lot of CFIT mishaps have occured with two or three pilot crews.
An excellent example was the G-III crash at Houston in November of 2004. Flown by a crew of 2, each of whom had considerable flight experience (over 19,000 hours each) and experience in type, one of whom was the current company chief pilot, and the other the former company chief pilot, both check airmen...descended into a parking lot during a routine ILS intercept enroute to pick up the former President of the United States. The nature of the mishap, an early descent into terrain below the published procedure, is somewhat similiar to the Seneca incident.
http://www.ntsb.gov/publictn/2006/AAB0606.pdf
The addition of a crewmember may be an enhancement to safety, but not necessarily so. A well equipped airplane with adequate performance does not prevent CFIT any more than a second crewmember, or an experienced crew.