FAR 61.57 "Recent Flight Experience", which contains the instrument currency requirements in question, only applies to the pilot-in-command. There is no reference to SIC instrument currency. As long as his/her annual recurrent training is current the FO is considered instrument current for the purposes of 121 operations, unless the individual operator has it's own currency requirements that are more stringent than the FARs.
Additionally, for pilots flying for operators who train under an Advanced Qualification Program, even the PIC only has to receive recurrent training once a year via a single 3-day simulator program under an exception to 61.57.
It is therefore possible to be current to fly instrument approaches in a 747 at night in storms as an airline FO under part 121 (or even Captain using AQP) while simultaneously being unable to fly as PIC carrying passengers under an IFR flight plan in a Cessna 172 on a VFR day under part 91.
Last edited by hikoushi; 5th December 2008 at 06:22.