can u elaborate a bit more why you wanna do this? Maybe my brain is too tired right now, but “normally” this makes no sense to me. You can “double” bones with ctrl-d, but they keep their hierarchy by this. Maybe there is another way in Blender doing what you want.
It doesn’t seem possible in the Outliner itself, plus you’re trying to parent a bone of an armature to a bone of another armature, which is not possible (you can give a Child Of constraint to a bone though, with the bone of another armature as Target)