Is DAL-A required for twin-engine helicopters with FADEC?
With no reversion to a direct mode, ie retaining control of the engine possibly with limitation after fadec failure would imply DAL A since that fault results in loss of ability to maintain flight. 

With some form of control reversion, the authorities would accept a much lower dal. 

You cannot claim credit for the second engine since both engines use common. Software and hardware. So any common cause, common mode fault could take out both engines. 

For part 29 helicopters  I would expect Dal A, part 27 probably B..