My experience has shown many helicopter are designed for single pilot operation, others can be flown by a single pilot but use two pilot crews, and in some EMS operations a single pilot aircraft will have a Medical Crew occupy the empty forward seat outbound to the pickup (some actually faced rearwards).
Instructor pilots have to be capable of flying a two pilot aircraft without assistance of the Student Pilots and sometimes inspire of the Student Pilots.
An example....the venerable Huey was designed for a ingle pilot in the righthand seat but was generally flown by two pilot in the Military but single pilot in the civilian world.
Then, when Vertical Reference flying (Long Line External Operation) the single pilot flew the aircraft from the Lefthand Seat.
Aircraft design plays a role in determining if it is a single or two pilot aircraft as well as the missions and SOP's of the Operator.
I never encountered a formal training course for Non-PIlot Crew Members to be able to assume full pilot functions although informal opportunities for them to have a go now and then did exist.
As to the Chinook.....it can be flown by a single Pilot without any other crew....why that would be done opens the discussion to lots of questions that would garner not many endorsing such.
In this day and time....could a 747 be flown by a single Pilot?
The question that has to be answered is if that single pilot becomes incapacitated for some reason....then what?
Why would one invite such a thing to happen by going to just the single pilot concept.
In single pilot helicopters I have been asked by. a. passenger what would happen if I died in flight.....and my usual reply was along the lines of you best hope your life insurance is paid up and your computer browser history has been deleted.