It's a feature of iOS 8 whereby other iOS devices and Mac computers on the same WiFi network can receive calls made to your iPhone. In your case this means that when someone calls or texts one of your iphones, it can also be received on the other iphone as well. If you don't want this feature you can shut it off by going to Settings>FaceTime>iPhone Cellular Calls and make sure the switch is slid to the left, turning it off.