[Biesmen]

You can use your Apple ID on multiple devices, so they are synchronised. I'm not sure if this works with an iPhone and iPhone, but it does work with an iPad and iPad (both my parents got their own iPad, and they use the same Apple ID). So if my father purchases an app, my mother can download it without paying.

Also, here is your answer from Whatsapp: http://www.whatsapp.com/faq/iphone/20888066