It will also depend on which jurisdiction you are logging for.
Some places call for logging of co-pilot, rather than second-in-command. Some regs refer only to in-seat, or stick time. Most that I have seen allow the PIC to log all time, whether in the bunk or not…
When I got to this stage, I recorded my time electronically according to my seat assignment. I then left it to the electronics to slice'n'dice depending on who was asking and for what purpose.
CM1 - assigned as PIC (except when under training)
CM2 - assigned as co-pilot, but not necessarily as 2IC (occasionally as PIC/RHS/Instructor)
CM3 - assigned as relief or Safety Pilot (as relief Captain, we would be 2IC; as would a jump-seat Safety Pilot for a trainee in RHS)
CM4 - never did this…, but it was an option on the fleet.
Under our NAA, both PIC & 2IC logged the whole flight. So I was always logging the whole flight, either PIC or SIC, or a bit of both!
…but with at least 6 different options this was too many columns!
Made do with 2 columns, PIC & SIC, plus 2 checkboxes (for Instructor & Relief)
Would recommend that you come up with a system that works for you, and stay consistent.
Good luck, fly safe.