The Ops Specs that I have seen really don't directly refer to the make up of a flight crew. 90% of Ops Spec are automatically generated by the issuing FSDO. There are some that the operator requests and some where the operator has some input to the wording (i.e. A008 Operational Control). My current experience is with a "basic" operator and previously with a small air ambulance and also with a scheduled 135 freight operator. Ops Specs for companies operating large, turbojet or transport category equipment may have additional insight. The requirement for an SIC operating IFR in an aircraft (regardless of aircraft certification) is contained in 135.101. Most operators with aircraft equipped with the proper autopilot apply for Ops Spec A015 in order to operate in lieu of an SIC for operational flexibility (in a BE58 it is almost a neccessity due to weight restrictions). This does not prevent an operator from using an appropriately qualified SIC when they have this authorization. It may be prudent to in the event the autopilot becomes inoperative at an out station. To return IFR, you need an SIC. In order to use an SIC, your training program (basic operator or above) needs to allow for SIC training procedures, the SIC must complete those training modules (your CRM and SOPs should be developed here), he will take a 135.293 checkride and be issued an 8410 that states he is SIC qualified. From his base month, his next training event will be in 12 mo (vs 6 mo. for PIC - 135.297). Ops Spec A007 will be issued (at least with a basic operator or less) to list by name PICs, SICs, Chief Pilot, etc. Your flight log will indicate the command position of each pilot. Your Operations Manual will also spell out the expected duties of PIC, SIC, etc. I haven't considered the 135 VFR scenario- don't care to. And when you dead head back on a Part 91 leg, I' sure that would start an arguement. I feel in that case the PNF would not have any legal loggable time unless you you explore a CFI/safety pilot scenario. There are legitimate uses, determined by regulation - FARs, Ops Spec, Inspectors handbook, for SICs and allows for the legal logging of that time. Any other pilot who does not meet the SIC requirements above, but who are "hired" / allowed to occupy a pilot position on a temporary request are nothing more than an observing passenger. PICs and operators must be very careful when allowing for this. Technically that "pilot" can't participate in or touch anything. The customer should also be briefed on this. There is a liability issue (even though it may be the insurance company requiring a second pilot). As far as the FAA is concerned, they would be very interested to find a participating SIC who is not qualified. Some understand the scenario and don't press the issue as long as that pilot is not an active participant in the flying duties.