In this case you'll want to use

set_reference_tasks

to a list of the tasks that you want to determine the final state of your flow. So you could so something like this:

@task
def task_one:
  #no failure here

@task
def task_two:
  #no failure here

@task(trigger=any_failed)
def set_failure() -> None:
    raise signals.FAIL()



with Flow("flow-name") as flow:
    task_1 = task_one()
    task_2 = task_two()

    failure = set_failure(upstream_tasks=[task_one, task_one])

flow.set_reference_tasks([task_1, task_2])

You can read a little more about that here: https://docs.prefect.io/core/getting_started/next-steps.html#reference-tasks