You can't start a second instance of an app. Or rather you can (run the app binary from the Terminal) but apps are not required to expect you to do this, and it

You can't start a second instance of an app. Or rather you can (run the app binary from the Terminal) but apps are not required to expect you to do this, and it would probably lead to data corruption from them writing to shared files. A weakness of this is you can duplicate apps and launch the duplicate, even though they have the same bundle ID, so they might still fight over things.