https://stackoverflow.com/questions/58651943/can-an-ansible-inventory-include-another/58657703?noredirect=1#comment135673227_58657703

session.txt

$ tree
.
└── inventories
    ├── children.yml
    ├── host.yml
    ├── otherhosts.yml
    └── shared.yml

1 directory, 4 files

$ cat inventories/children.yml 
---
testgroup:
  children:
    group1:
    sharedservers:

$ cat inventories/host.yml 
---
group1:
  hosts:
    host1:
    host2:


$ cat inventories/otherhosts.yml 
---
group2:
  hosts:
    hostA:
    hostB:


$ cat inventories/shared.yml 
---
sharedservers:
  hosts:
    host3:
    host4:


$ ansible-inventory -i inventories/ --list
{
    "_meta": {
        "hostvars": {}
    },
    "all": {
        "children": [
            "ungrouped",
            "testgroup",
            "group2"
        ]
    },
    "group1": {
        "hosts": [
            "host1",
            "host2"
        ]
    },
    "group2": {
        "hosts": [
            "hostA",
            "hostB"
        ]
    },
    "sharedservers": {
        "hosts": [
            "host3",
            "host4"
        ]
    },
    "testgroup": {
        "children": [
            "group1",
            "sharedservers"
        ]
    }
}

$ ansible -i inventories/ testgroup -m debug -a var=inventory_hostname
host4 | SUCCESS => {
    "inventory_hostname": "host4"
}
host1 | SUCCESS => {
    "inventory_hostname": "host1"
}
host2 | SUCCESS => {
    "inventory_hostname": "host2"
}
host3 | SUCCESS => {
    "inventory_hostname": "host3"
}