With a “Layerd blend per bone” Node in the AnimGraph of your AnimationBlueprint.
Connect the “moving” Animation with the “base pose” pin and your duplicated one with the “blend poses 0”. Blend Weights should be “1” for the full effect.
Dont Forget do specifiy the bone, that seperates the normal Animation with your duplicated one (LayerSetup -> 0 -> Branch Filter (Add one element) -> Bone Name (specify it here))
If you are using the Default epic Mannequin, i suppose you should use the bone “spine_01” or “spine_02”.