Back when Boeing was developing the 767 (~1978-82) the feds were doing a big study regarding the safety of two vs. three flight decks. Conventional wisdom was the three were better than two - if nothing else an extra set of eyes monitor gauges and look for other aircraft - so aircraft over a certain size needed three crew. As a result, the baseline 767 design was for three crew - with round dials/gauges and a proper flight engineer station - while the two crew EICAS equipped flight deck was pretty much on the back burner. But the results of the study surprised pretty much everyone - that the two crew was not only as safe as a three crew, it was actually somewhat safer. Basically that the third person tended to add distraction/confusion factors that outweighed the advantages of having another person (I suspect the relative lack of proper CRM at the time played a role in this finding). The airlines that had ordered the 767 took one look at the report, and asked Boeing 'how much to switch to the two crew EICAS flight deck?" (IIRC, it was ~$500,000, which the airlines figured they could save in less than two years by not having a flight engineer). At the time, the first few 767s were already being assembled with the 3 crew flight deck - the very first one to fly (VA001) was left in the 3 crew configuration for first flight, while all the others were converted to 2 crew with EICAS before rollout (VA001 was eventually retrofit to the EICAS configuration after the certification flight test program was finished). There was one or two operators who still wanted a flight engineer station (Ansett comes to mind but don't quote me) so Boeing ginned up a simple FE station, but the rest of the flight deck was still the 2 crew EICAS configuration - the three crew non-EICAS configuration was never delivered.
From the designer point of view, two crew meant we had to pay much more attention to crew workload. For example, throttle stagger - on the 747 classics, no one cared about throttle stagger - some engine intermix configurations could have well over a knob of stagger because you had a FE to constantly tweak the throttles to align EPR or N1. On the 767/757 we had strict limits on the amount of throttle stagger - less than a quarter knob (increased to 1/2 knob for certain fault conditions).