- hosts:
    - testhost
    - testhost2
  gather_facts: no
  vars:
    secondhost: testhost2
  tasks:
    - name: Call the first role only on one host
      include_role:
        name: role1
      when: inventory_hostname is match(secondhost)

    - name: Make sure nothing else runs until role1 finishes
      assert:
        that:
          - "'role1_complete' in hostvars[secondhost]"