Assuming you have two instances of an application open, you can press Cmd + ~ to switch between them.

This is especially useful for scenarios like:

• You have two projects open in PHPStorm – one that’s a Symfony API, one that’s a React front-end.
• You have two instances of Chrome running. One where you’re logged in to an app as an admin user, one where you’re logged in as a normal user. (This could be especially useful for using Jetbrain’s JavaScript debugger)