How to configure TeamCity to run the tests concurrently rather than one by one?
The best way to do this is to add more build servers. You cant run multiple instances in interactive mode as they will crosstalk
More Build Agents can execute build configurations concurrently.
The build configurations are executed by Build Agents. It is very likely that you have one on the build server, where TeamCity is running. You can add another one on another machine. As far as I know it is not possible to have more than one Build Agent on one machine.