V14!

New workflow!

Rig your mesh onto the StepOne blendfile (you may now use automatic weight paint!), then Append the StepTwo Armature and set your hind.chest as a child of your new Humanoid Hips!

There's PLENTY OF NUANCE in how exactly to go about it, but that is the new workflow... I recommend you zero the weights of the hind.spine vertex group, or lock it prior to running the auto weight paint, as hind.spine should not be weighted. Make sure your viewpoints match--but you might want to scale and adjust your StepOne rig to fit the StepTwo armature's viewpoint, since this allows you to copy the project's existing Avatar Definition.