Start two instances of IntelliJ IDE

CrazyCoder has roughly the right idea. However, setting the config file alone was not sufficient for me to run multiple instances. Here are my steps to get this going (in GNU/Linux, I am sure you can figure out equivalent in other systems):

1. Create a folder/directory per instance you want to run.

   mkdir -p ~/idea/instance-0
   

2. Go to the installation directory (e.g. /opt/intellij) and copy the idea.properties (in bin) file over to your instance directory.

   cp /opt/intellij/bin/idea.properties ~/idea/instance-0/
   

3. Copy 3 more directories: system, plugins, and config. I highly recommend doing this without the running instance

   cp -r /opt/intellij/system ~/idea/instance-0/
   cp -r /opt/intellij/plugins ~/idea/instance-0/
   cp -r /opt/intellij/config ~/idea/instance-0/
   mkdir ~/idea/instance-0/log
   

4. Open your idea.properties file and update the configurations for your directories:

   #---------------------------------------------------------------------
   # Uncomment this option if you want to customize path to IDE config folder. Make sure you're using forward slashes.
   #---------------------------------------------------------------------
   idea.config.path=${user.home}/config
   #---------------------------------------------------------------------
   # Uncomment this option if you want to customize path to IDE system folder. Make sure you're using forward slashes.
   #---------------------------------------------------------------------
   idea.system.path=${user.home}/system
   #---------------------------------------------------------------------
   # Uncomment this option if you want to customize path to user installed plugins folder. Make sure you're using forward slashes.
   #---------------------------------------------------------------------
   idea.plugins.path=${user.home}/plugins
   #---------------------------------------------------------------------
   # Uncomment this option if you want to customize path to IDE logs folder. Make sure you're using forward slashes.
   #---------------------------------------------------------------------
   idea.log.path=${user.home}/log
   

5. Now, you can start IntelliJ with the new setup:

   IDEA_PROPERTIES=~/idea/instance-0/idea.properties /opt/intellij/bin/idea
   

Obviously, you probably want to put the command in a script file for invocation. This seems to work for me.

Press Ctrl+Alt+S

Choose Appearance & Behavior, then System Settings, check radio button: Open project in new window.

You need to configure each instance to use its own folders for config/plugins/system locations by editing idea.properties file on Windows/Linux and Info.plist on Mac. You can find the details in FAQ.

Note that normally it's not necessary since you can open multiple projects in different IDEA frames within the same instance using File | Open or Open Recent.