Hi Anton,

I did not feel like debugging your docker-compose file, but I could not find any test covering your scenario on github/janusgraph either, so I just replayed your scenario with the default janusgraph-full-0.5.3 distribution. These are the steps:

1. start a cassandra-cql instance with bin/janusgraph.sh start   (ignore the gremlin server and elasticsearch that are started too)
2. make two files conf/gremlin-server/gremlin-server-configuration8185.yaml and conf/gremlin-server/gremlin-server-configuration8186.yaml, using conf/gremlin-server/gremlin-server-configuration.yaml as a template but changing the port numbers,
3. start two gremlin server instances with these yaml files, so serving at port 8185 and 8186
4. make two files conf/remote8185.yaml and remote8186.yaml
5. start two gremlin console instances and play the following:

In the first console:
gremlin> :remote connect tinkerpop.server conf/remote8185.yaml session
==>Configured localhost/127.0.0.1:8185-[3aa66b8e-8468-4cd7-95aa-0e642bb8434c]
gremlin> :remote console
==>All scripts will now be sent to Gremlin Server - [localhost/127.0.0.1:8185]-[3aa66b8e-8468-4cd7-95aa-0e642bb8434c] - type ':remote console' to return to local mode
gremlin> map = new HashMap<String, Object>();
gremlin> map.put("storage.backend", "cql");
==>null
gremlin> map.put("storage.hostname", "127.0.0.1");
==>null
gremlin> map.put("graph.graphname", "graph1");
==>null
gremlin> ConfiguredGraphFactory.createConfiguration(new MapConfiguration(map));
==>null
gremlin> graph1 = ConfiguredGraphFactory.open("graph1")
==>standardjanusgraph[cql:[127.0.0.1]]
gremlin> g1 = graph1.traversal()
==>graphtraversalsource[standardjanusgraph[cql:[127.0.0.1]], standard]
gremlin> g1.addV()
==>v[4136]
gremlin> g1.V()
==>v[4136]
gremlin> g1.tx().commit()
==>null
gremlin>

In the second console:
gremlin> :remote connect tinkerpop.server conf/remote8186.yaml session
==>Configured localhost/127.0.0.1:8186-[00729ace-48e0-4896-83e6-2aeb19abe84d]
gremlin> :remote console
==>All scripts will now be sent to Gremlin Server - [localhost/127.0.0.1:8186]-[00729ace-48e0-4896-83e6-2aeb19abe84d] - type ':remote console' to return to local mode
gremlin> graph2 = ConfiguredGraphFactory.open("graph2")
Please create configuration for this graph using the ConfigurationManagementGraph#createConfiguration API.
Type ':help' or ':h' for help.
Display stack trace? [yN]n
gremlin> graph1 = ConfiguredGraphFactory.open("graph1")
==>standardjanusgraph[cql:[127.0.0.1]]
gremlin> g1=graph1.traversal()
==>graphtraversalsource[standardjanusgraph[cql:[127.0.0.1]], standard]
gremlin> g1.V()
==>v[4136]

The assignment to graph1 differs from what is shown in the ref docs at:
https://docs.janusgraph.org/basics/configured-graph-factory/#binding-example

But otherwise the scenario you are looking for works as expected. I trust you can use it as a reference for debugging your docker-compose file.

Best wishes,    Marc