Have operated DH6 aircraft in the past (first in1966) both single and multi-crew...it definately is NOT a multi-crew aircraft, it only 'becomes' one when the regulatory authorities demand same for the type of operation.
Lets face facts here.
Dead simple aeroplane, altho very reliable.
Examples of multi crew aeroplanes would be...F27, HS748, DH8...well the list goes on.
Having said this, the respective airlines' requirement for 'former' multicrew ops is nonsense. Any competent pilot can be trained for a first officer position, even with low hours. It is not rocket science.
Suspect the respective airline has poor trainers/check pilots.
They really should pull up their collective socks and do the job...right.