stub.yml

---
things:
  one:
    b: b
    x: x
    z: z
  two:
    b: something
    x: x
    z: z

template.yml

---
things:
  <<: (( merge || nil ))
  one:
    a: a
    b: (( merge ))
    c: c
    d: d
    e: e
    f: f
  two:
    a: a
    b: (( merge ))
    c: c
    d: d
    e: e
    f: f