KIF runs in-process for your app, and the "Back to <Other App>" button at the very top left is part of the status bar view, which I'm fairly certain is owned by a different OS process. So, in short, it's unlikely that you can tap that button.
In general, because of limitations from being run in-process, KIF isn't appropriate for testing things like multi-app switching, backgrounding, or similar scenarios that have to interact with Springboard or other parts of the external OS. In your scenario, even if you could tap the "Back to" button, KIF wouldn't have access to the app you switched to, so you'd be stuck there.