You first ask whether it would be proper under FAR 61.51(g) for a properly qualified SIC to log instrument flight time flown during instrument conditions while serving as the SIC in Part 121 operations on an aircraft that requires two crewmembers. The answer is yes. As a qualified SIC, and as a required crewmember, you are "operating" the aircraft within the meaning of FAR 61.51(g). Therefore, as the SIC operating the aircraft "solely by reference to instruments under actual or simulated instrument flight conditions," you would log that time as SIC flown in instrument conditions. Naturally, the PIC logs the time as PIC flown in instrument conditions.