Greetings,
When searching for example/questions on how to do multiple cameras in Phaser, I came up with only one answer: using multiple Phaser instances. That was not how I wanted things to be done, so I made my own variant on it (which I saw Rich made a comment about on how it could be done) with a RenderTexture.
I made a repository on GitHub containing a (fairly) heavily commented example on how things could be done this way, plus a live example. I'm honestly not sure about performance, but I can't imagine it would be worse than having multiple instances of Phaser.
Look through the example, clone the repository and modify it to your heart's content, and if you have any questions, I'll do my best to answer them.
This example was tested with Phaser CE 2.7.3
-MikauSchekzen