Start two instances of IntelliJ IDE
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.