How do I control two armature animations in one gameObject? (Mecanim)

Hi all - as usual I've searched for the answer and can't find it, apologies if I'm repeating someone else...

I've imported an aircraft model into Unity from Blender, with two separate armatures - one to control the doors, and one for the undercarriage of my aircraft.

Here's some screenshots, so you can see what I'm doing, and hierarchies etc...

Before I built the undercarriage (just one armature), I had the Animator on the parent gameObject (newChopper2), and it worked fine. Now I've added the undercarriage and its armature and I don't know how to get the animation to play.

I tried adding a second Animator to the undercarriage gameObject (gear), but I get the message "Animator has not been initialised".

I also tried adding a new layer to the existing controller, but I'm not experienced with layers, so don't know if I was hashing it up, or it's just not suitable.

I know I could make it one armature for both items, but I want them to function independantly, gear up and doors open or gear up doors closed, both together etc.

Can anyone tell me what I'm doing wrong please or a way to do what I want? Thanks.

Ian