How to get text messages forwarded to me without the iPhone owner knowing?

I have 2 iPhones in my name through Vodaphone Australia. I was wondering if there's any ways to be able to get messages sent to and from my second iphone onto my first iphone without the user of the second iphone realizing?

I just want to know if it is possible and how to forward the messages to my iphone. No comments on why I would do this and how wrong it is snooping into others privacy please.

No, you could use the same apple id for that purpose but they would know.