Both the UK and Caribbean crash are good examples of pretend multi-crew ops and inadequate procedural familiarity and training for the type of aircraft and operation (a confined area take-off at night in fog - a truly daft idea; an ad hoc night departure into a no external reference environment without applying a recognised procedure).
Paper-ticking instrument currency and two bods on seats is nowhere near the level needed to have avoided those accidents. And having an extra bod in with the pilot in the crash in question would have been part of an inculcation into an operations culture which would lead to an accident that day or another.