Yes it should be possible.
This is strange, because when the second session boots, if it decided to take the same port, it would likely just fail, given you can’t bind to the same port. So why would it be the case that it seemingly booted correctly?
You may have to attach logs for both the first and second sessions. Specifically the log lines that say what host/port it’s binding to. We randomly generate a port to bind to, so it should be the case.
Also, when you say ‘default rstudio app’ do you mean our example here?. Have you modified any of the parts that allocate ports, or write the connection.yml file?