Kick the tyres,
Captplaystation is right.
Move the landing gear lever, autobrake selector and a few other things to the left hand side of the pedestal and a 737 could be flown just as easily single crew as a king air or a citation 1.
Having got the two pilots we can then invent CRM and all the good monitoring stuff that we all do, and hopefully say to the F/O "which sectors would you like?" and then let him or her make all the decisions, helping them along the ardous road to the left hand seat which some of us travelled via FI and air taxi and turboprop F/O then jet F/O and some travelled by SSTR straight to a jet.
But the underlying reason for having two pilots on a public transport flight remains the same as the reason for having two engines - in case one of them dies.