I've recently revived the PR Fredrik Hedberg did a long time ago for dual helicopter support in ArduPilot to get it ready for merging into master. Recent changes in the structure of ArduPilot makes supporting these vehicle types much easier.
Here is a video of flying the Chinook model in RealFlight using ArduPilot SITL
The PR for dual heli support is here:
I look forward to some people trying this on a real Chinook style heli!