I should add in all fairness that John Lynch, who writes much of FAR 61 and maintains the FAA AFS-640 FAQ web site, takes the opposite stance of the information I have given. In his FAQ on the subject, he states that the PNF should not log instrument time.
The FAQ site is not endorsed by the FAA and is not regulatory, and for the most part, it provides an element of clarity for the regulations. There are areas of general disagreement, and I have disagreed on several points in the past with what is printed there.
Mr. Lynch states that instrument time logged as SIC/PNF is not useable for the purposes of currency (the response was written when 6 hours were still required for currency), and is not valid for the purposes of meeting the instrument time requirements for the ATP certificate. I agree in substance to both these claims, as the purpose of each is to provide actual instrument experience, and speaks to a different purpose in logging.
However, for logging instrument time, the fact remains that one is not logging experience with respect to manipulation of the controls in reference to instruments, but conditions of flight.
Mr. Lynch also states that he personally believes that instrument conditions in legal VFR (dark moonless night over the ocean, with legal separation from clouds, for example) should be listed as simulated instrument flight. I disagree, though by most counts, my view on that subject would be a losing cause. (As far as I am concerned, flight requiring reference to instruments represents instrument conditions, and absent a view limiting device, represents actual instrument conditions, while not necessarily instrument meteorological conditions).
Interestingly, Mr. Lynch also takes a potentially controversial stance that one does not require a safety pilot in conditions mentioned above, as a view limiting device is not used, although he states that such conditions represent simulated, and not actual instrument conditions. To each his own. I don't see his version being particularly defensible, except that it most likely represents the viewpoint at-large for the majority of inspectors.