This app is not required on both the phones, just send message code(can be seen in app) as sms to other android phone which have this app running and which have made your number as authorized number, and you will get call back from that number.
There can be numerous uses of this app:-
~ Getting call from number of your loved one to know if they are okay.
~ Emergency call from other phone and hear what is going on.
~ Other party not picking your phone, (busy or not hearing ring tone) just send sms and get call.
~ Plenty other uses.