Hushed is a similar app. You can use it to register a number for a week and then use that number to verify whatsapp. Some android phones allow you to have a second whatsapp and some samsung phones also have a secure folder where you can make the folder invisible or disguise it with any icon or name and place a separate passcode/biometric lock on it. You can even set up a separate contacts list in this phone, which isn't accessible from outside the folder. This basically gives you a second, secure phone within your phone into which you can place your 2nd whatsapp with its own number. It removes the need for a 2nd punting phone and gives you a good level of privacy and security - enough that you can hand your phone to your other half and not worry.