This flow will start the two sequential tasks in parallel and those will launch tasks one after the other.

yaml
id: sequential
namespace: io.kestra.demo

description: |
  This flow will start the 2 sequential tasks in parallel and those will launch tasks one after the other.

tasks:
  - id: parent
    type: io.kestra.core.tasks.flows.Parallel
    tasks:
      - id: seq1
        type: io.kestra.core.tasks.flows.Sequential
        tasks:
          - id: t1
            type: io.kestra.plugin.scripts.shell.Commands
            runner: PROCESS
            commands:
              - 'echo "running {{task.id}}"'
              - 'sleep 1'
          - id: t2
            type: io.kestra.plugin.scripts.shell.Commands
            runner: PROCESS
            commands:
              - 'echo "running {{task.id}}"'
              - 'sleep 1'
          - id: t3
            type: io.kestra.plugin.scripts.shell.Commands
            runner: PROCESS
            commands:
              - 'echo "running {{task.id}}"'
              - 'sleep 1'
      - id: seq2
        type: io.kestra.core.tasks.flows.Sequential
        tasks:
          - id: t4
            type: io.kestra.plugin.scripts.shell.Commands
            runner: PROCESS
            commands:
              - 'echo "running {{task.id}}"'
              - 'sleep 1'
          - id: t5
            type: io.kestra.plugin.scripts.shell.Commands
            runner: PROCESS
            commands:
              - 'echo "running {{task.id}}"'
              - 'sleep 1'
          - id: t6
            type: io.kestra.plugin.scripts.shell.Commands
            runner: PROCESS
            commands:
              - 'echo "running {{task.id}}"'
              - 'sleep 1'
  - id: last
    type: io.kestra.core.tasks.debugs.Return
    format: "{{task.id}} > {{taskrun.startDate}}"