Application to send sms sent to one mobile device to the other

[Ajeenahknj]

I've been told there is an app that you can put on someones phone and any text messages that are sent to that phone is automatically sent to your phone at the same time. You put this app on the persons device and it disappears so they don't know it's there and you have enter a password in order for it to pop up. Please help if you know what I'm talking about, thank you.

Sincerely, Frustrated