How to run multiple roscore/master in a single machine?

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Hi,

A superset question is here, where it is said that we have to change the ROS_MASTER_URI to run the 2nd master.

I have tried running the 1st master with ROS_MASTER_URI=http://localhost:11311 (default), then in another terminal did the following:

$ export ROS_MASTER_URI=http://torPC2:11315/
$ echo $ROS_MASTER_URI
http://torPC2:11315/
$ roscore
...
roscore cannot run as another roscore/master is already running. 
Please kill other roscore/master processes before relaunching.
The ROS_MASTER_URI is http://torPC2:11311/

So, how to make roscore know that the ROS_MASTER_URI now in the 2nd terminal is http://torPC2:11315/ ? or How to run multiple roscore/master in a single machine?

My use case requires completely independent roscores/masters and even they do not need to communicate each others. Simply, it is like having multiple isolated machines with one running roscore/master in each of them.

Thanks a lot in advance.