ansible shell command only if another shell command does the correct thing when looping over a list

gistfile1.txt

vars:
drbdmanage_m:
  volumes:
    - name: server1_disk1
      size: 30G
    - name: server1_disk2
      size: 30G

task:

---
- name: drbdmanage_check_volume_exists_{{item.name}}
  shell: "drbdmanage list-volumes"
  register: existing_drbdmanage_volumes
  with_items:
    - "{{ drbdmanage_m.volumes }}"
  when:
    - inventory_hostname in [ drbdmanage_deploy_node ]

- name: drbdmanage_add_volume_{{item.name}}
  shell: "drbdmanage add-volume {{ item.name }} {{ item.size }}"
  with_items:
    - "{{ <<??? yeah which items and must registered var "existing_drbdmanage_volumes" be used here also ???>> }}"
  when:
    - "{{ <<??? how to check registered var "existing_drbdmanage_volumes" correctly ???>> }}"
    - inventory_hostname in [ drbdmanage_deploy_node ]