If you are referring to 61.51(g)(1), which states in part, "A person may log instrument ime only for that flight time when the person operatoes the aircraft...", then the answer to your question is clear. Unless two people share a logbook, the wording must be in the singular.
The regulation is referring to the logging of flight time, not to the operation of aircraft. Clearly an aircraft requiring two crewmembers to operate, is operated by two crewmembers. However, each crewmember logs that time separately. Therefore, as the regulation speaks to the individual airman, it is written in the singular.
The regulation referring to the requirement for more than one crewmember is legion, and begins with the type certification for the aircraft, includes the operations specifications and airworthiness limitations, and may include other parts of 14 CFR. Unless otherwise approved by the administrator, an aircraft requiring more than one crewmember is operated by each required crewmember.
A SIC in instrument conditions is in them every bit as much as the PIC. The regulation does NOT differentiate between SIC and PIC instrument time. An employer may ask for a breakdown, but legal logging of time in accordance with 14 CFR 61.51 only requires the logging of instrument time. This time may be further broken down, by definition, into actual or simulated instrument time.
A SIC may log the conditions of flight. To do so would be to ignore the fact that the SIC performed the duties of SIC and operated the aircraft as SIC, in instrument conditions, or at night.
One might postulate that because the SIC didn't perform a landing, he or she is not entitled to show arriving at LAX. Instead, only the PIC would be allowed to log the destination. This is obviously ridiculous. Both parties are there, both parties made the flight; both parties identify the location to which they flew, in their logbooks. Likewise, if both parties entered instrument conditions or flew at night, then both parties log the conditions of flight, as dictated in 61.51(b)(3).
Only the pilot performing a landing or an approach logs that landing or approach. Otherwise, both pilots log the conditions of flight.