PICUS
A) you have an ATPL: log hrs as SIC while under supervision (or as PIC)
Only one pilot is assigned PIC in the TechLog or GD..., so only one pilot can log the hours as PIC (of course with the exemption of LPC checkflights on the aircraft)
B) you have no ATPL and the company assigns you to fly as PICUS: you log the hrs as PICUS (but not as PIC, as to my knowledge) - again, there is only one PIC marked in the TechLog.
my2cents
welle