Advice on a wood chopping clip

[Targos]

Hi there
I've got a mocap clip, where I'm chopping wood, but the movement between hands is a little bit sloppy, as though I had a rubber handled axe. I'd like to constrain one hand to the other so that one is driving the IK handle of the other, but I'm having trouble overriding the "child" arms animation. Does anyone have any advice in how I would set this up? I also need to be able to keyframe its influence so that the hands can separate for parts of the animation and be driven by existing keyframes. Thanks a bunch
AaronC