How do I put video A on top of video B? About merge videos on iOS

Question

I want to merge two videos, it look likes:

• Video A:

• Video B:

• Last video:

This is a very interesting deal, you could try this on app named SHOOT.I want to implement a similar effect, but how? Can GPUImage or AVAnimator help me implement it?

I had try GPUImage like this:

let movieURL = URL(
    string: "sample_iPod.m4v",
    relativeTo: Bundle.main.resourceURL!
    )!
let specialMovieURL = URL(
    string: "cartoon_watermark.mov",
    relativeTo: Bundle.main.resourceURL!
    )!
let writerURL = URL(
    fileURLWithPath: NSTemporaryDirectory().appending("last.mov")
)
print("Writer URL: \(writerURL)")

do {
    try FileManager.default.removeItem(at: writerURL)
} catch let error {
    print(error.localizedDescription)
}

self.movie = GPUImageMovie(url: movieURL)
self.movie.delegate = self
self.movie.playAtActualSpeed = true

self.specialMovie = GPUImageMovie(url: specialMovieURL)
self.specialMovie.playAtActualSpeed = true

self.filter = GPUImageScreenBlendFilter()

self.movie.addTarget(self.filter)
self.specialMovie.addTarget(self.filter)
self.filter.addTarget(self.renderView)

self.movie.startProcessing()
self.specialMovie.startProcessing()

It didn't works...Or if I could use AVFoundationKit to get it done?

Thank you for your answer :)