You will need to start the Modeler twice (or more times) and use a different port number for each application.
Yes, as stated in the in the how to link, you can specify different port numbers to run multiple apps locally at the same time. What do you mean by:
but once you run a project it automatically closes the other.
Are you running on the same database?
Change the admin port for each project.
Project > Setting >Configurations tab, Edit > Server tab > change the admin port > ok > run