which command to execute to run multiple OLAP queries?
You can use i.e. GNU Parallel which provides possibility to run arbitrary commands/programs at the same time, the syntax would be something like:

parallel psql -h localhost -d databasename -U user -p port -a -q -f input{}.sql -o output{}.txt ::: 1 2


this command will execute 2 queries located in input1.sql and input2.sql at the same time. 

Another option would be going for a dedicated load testing tool like Apache JMeter, if you add Postgres JDBC Driver to JMeter Classpath you will be able to provide your connection details under JDBC Connection Configuration and the query(ies) in the JDBC Request Samplers. Check out The Real Secret to Building a Database Test Plan With JMeter article for more details if needed. 

Then you can increase the number of threads in Thread Group and JMeter will execute the queries according to your workload model. You will be able to observe performance metrics either using Listeners or via HTML Reporting Dashboard. This way you will get more comprehensive results and be able to correlate increasing number of concurrent threads with increased queries execution time, identify saturation points, bottlenecks, etc.