FYI, some 2 engine A/C had provisions for 3 crew members. Originally, United ordered all of their first B-737s with an engineer's seat. I have yet to see one, and not sure if any are around any longer.
The reason for the extra member was due to the fact the UA was hiring pretty low time pilots then (lol, ya "then") and thought it to be benificial to have another body up front. When it comes to decision making, it's always good to have a "tie-breaker!" I have to admit that I prefer a 3 man crew, just not in the confines of a 73!