Let say we have the two docker-compose files in separate directories which we want to link together.

First, we have:

version: '2'
services:
    A:
        image: nginx

It is important to know the specific network name (external_name) to be used by the first docker-compose up. We’ll need to run the first with -p to define the project name which is a part of the network name in {project_name}_default fashion.

In this case, run docker-compose -p A up it will automatically create A_default as a default network for the compose.

Second, we have:

version: '2'
networks:
    A_network:
        external:
            name: A_default
services:
    B:
        image: debian
        command: bash
        networks:
            - default
            - A_network

Now, run it, this time I will just run docker-compose run B for the purspose of accessing the shell.

And try, pinging A (across the network).

[email protected]:/# ping A
PING api (172.21.0.2): 56 data bytes
64 bytes from 172.21.0.2: icmp_seq=0 ttl=64 time=0.101 ms
64 bytes from 172.21.0.2: icmp_seq=1 ttl=64 time=0.072 ms
64 bytes from 172.21.0.2: icmp_seq=2 ttl=64 time=0.072 ms
64 bytes from 172.21.0.2: icmp_seq=3 ttl=64 time=0.073 ms

Understand that, B also has its own default network, but it also taps to the external one as defined in A_network which ultimately connects to the A_default external network created by the first compose.