Example using generated workspace subpaths and interpolation to retrieve the value, absolute paths in the Tasks, two separate workspaces instead of the hierarchical mailbox workspace

volumes.yaml

apiVersion: tekton.dev/v1alpha1
kind: Task
metadata:
  name: make-stuff
spec:
  params:
  - name: message
  steps:
  - name: doit
    image: alpine
    command: ["/bin/sh", "-c"]
    args:
      - echo $(params.message) > /workspaces/messages/message;    
  workspaces:
    - name: messages
      description: the folder where we write the message to
---
apiVersion: tekton.dev/v1alpha1
kind: Task
metadata:
  name: read-messages
spec:
  steps:
  - name: doit
    image: alpine
    script: |
      #!/usr/bin/env bash
      cat /workspaces/hello-mailbox/message
      cat /workspaces/world-mailbox/message
  workspaces:
    - name: hello-mailbox
      description: the folder containing the hello message
    - name: world-mailbox
      description: the folder containing the world message
---
apiVersion: tekton.dev/v1alpha1
kind: Pipeline
metadata:
  name: volumes-forevah
spec:
  volumes:
  - name: the-volume
    description: the volume to put everything on
  tasks:
  - name: write-hello
    taskRef:
      name: make-stuff
    inputs:
      params:
      - name: message
        value: hello
    workspaces:
    - name: messages
      volume: the-volume
  - name: write-world
    taskRef:
      name: make-stuff-world
    inputs:
      params:
      - name: message
        value: world
    workspaces:
      name: messages
      volume: the-volume
  - name: read-it-all
    taskRef:
      name: read-messages
    workspaces:
    - name: hello-mailbox
      volume: the-volume
      subPath: $(tasks.write-hello.workspaces.messages.subPath)
    - name: world-mailbox
      volume: the-volume
      subPath: $(tasks.write-world.workspaces.messages.subPath)
---
apiVersion: tekton.dev/v1alpha1
kind: PipelineRun
metadata:
  name: volumes-forevah-run
spec:
  pipelineRef:
    name: volumes-forevah
  volumes:
  - name: the-volume
    persistentVolumeClaim:
    - name: my-pvc