I think the easiest way to understand what to log is:
PIC is for the person who is given the responsibilty as the commander/captain.
SIC is for the other person.
It doesnt matter who is pilot flying/handling pilot, it is what position/rank/level of responsibility you have been given on the plane that determines what you log.
For example, if there were 2 ATPL pilots flying the 1900, one would be dedicated as the captain, the other the fo, and only the captain could log PIC time and the other guy SIC.
Thats how it works under JAR.