Part 135 instrument currency in a jet aircraft requiring two pilots, requires both pilots to be instrument current. But how about the same situation for a Part 91 flight? Does the SIC need to have his 6 approaches with tracking, intercepting, holding etc... in the last 6 months?
Only if you fall under the Fractional rules (Subpart K). Otherwise SIC currency requirements for turbojet airplanes are specified in 14 CFR 91.531, which requires you to be current via 14 CFR 61.55 and has no instrument currency requirement (although you do need your instrument rating to fly under IFR).