file1.yaml

---
kind: MutatingWebhookConfiguration
metadata:
  name: defaults.webhook.kpack.io
webhooks:
  - name: defaults.webhook.kpack.io
    tomato: sticks

file2.yaml

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

#@ def matching():
kind: MutatingWebhookConfiguration
metadata:
  name: defaults.webhook.kpack.io
#@ end

#@overlay/match by=overlay.subset(matching()), expects="0+"
---
webhooks:
  #@overlay/match by="name"
  - name: defaults.webhook.kpack.io
    #@overlay/match missing_ok=True
    fish: b