ytt templating, how to combine values from two yamls into third? NOT WORKING, HELP NEEDED.

config.yaml

#@ load("@ytt:data", "data")

metadata: #@ data.values.cluster_metadata
platform: #@ data.values.platform

demo-secrets.yaml

#@data/values
--- 
platform: 
  vsphere:
    username: "username-value"
    password: "password-value"
pullSecret: "pullSecret-value" 
sshPubkey: "ssh-rsa pubkey value"

demo-values.yaml

#@data/values
--- 
cluster_metadata:
  name: demo

expected.yaml

metadata:
  name: demo
platform:
  vsphere:
    username: username-value
    password: password-value

result.txt

$ ytt -f config.yaml -f demo-secrets.yaml -f demo-values.yaml 
ytt: Error: Overlaying data values (in following order: demo-secrets.yaml, demo-values.yaml, additional data values):
Document on line demo-values.yaml:2: Map item (key 'cluster_metadata') on line demo-values.yaml:3: 
Expected number of matched nodes to be 1, but was 0

I'm sorry to say that https://get-ytt.io/#example:example-multiple-data-values has not helped me. 
I have tried to pepper the demo-*.yaml files with #@overlay/match missing_ok=True where I think they could relevantly 
go, to no avail.