The price for laziness is a poor face detection algorithm. Among many other face swap apps out there, the developers of this particular one decided that people are too dumb to, crop, resize, rotate or skew an image. So they put together an app that does everything for you!
How great you might think. Except it doesn't work. The app cannot detect faces. Even if it implies that it has detected one face, without having detected the other, there's no deal for you. You can imagine the lottery of processing a photo with three or more faces in it...
So in the end, just so you wouldn't say you came here for nothing, here's an edit that I've made using a DIFFERENT app:
Automatic face swapping
Let's just say: [Blocker] [OPEN] - Face detection feature does not perform as required.
Face swapping is a rather popular thing, so naturally there are many apps that do this. Just search around for the one that you like best. Ciao.