how to merge two video with transparency

I have successfully merge video-1 and video-2, over each other with video-2 being transparent using AVFoundation framework but after merging below video(video-1) is not displayed only video-2 is visible but when I use below code

AVMutableVideoCompositionLayerInstruction *SecondlayerInstruction =[AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:secondTrack];
[SecondlayerInstruction setOpacity:0.6 atTime:kCMTimeZero];

its set opacity on video-2 layer.But here actual problem is, there are some content over video-2 layer which is not transparent and here after applying opacity over video-2 layer it also apply over that content which is not transparent.
I am adding two image here which describe both scenario after set opacity using AVMutableVideoCompositionLayerInstruction

• as in image after merging transparent area is black and when I set opacity over second layer whole the video-2 goes transparent now but the content also become transparent.
• but my question is that how to play transparent video over another video after merging.I already checked video-2 is transparent as it proper play in android platform.

Edited-1 : I also try to set a background color on myVideoCompositionInstructionwhich also not helped. taking reference from this old question link

Edited-2 : In AVVideoComposition.h, I found

Indicates the background color of the composition. Solid BGRA colors only are supported; patterns and other color refs that are not supported will be ignored. If the background color is not specified the video compositor will use a default backgroundColor of opaque black. If the rendered pixel buffer does not have alpha, the alpha value of the backgroundColor will be ignored.

What it means, I didn't get it.can any one help?

Good Question :

Try This

var totalTime : CMTime = CMTimeMake(0, 0)

func mergeVideoArray() {

let mixComposition = AVMutableComposition()
for videoAsset in arrayVideos {
    let videoTrack = 
        mixComposition.addMutableTrack(withMediaType: AVMediaTypeVideo, 
                                       preferredTrackID: Int32(kCMPersistentTrackID_Invalid))          
    do {
        if videoAsset == arrayVideos.first {
            atTimeM = kCMTimeZero
        } else {
            atTimeM = totalTime // <-- Use the total time for all the videos seen so far.
        }
        try videoTrack.insertTimeRange(CMTimeRangeMake(kCMTimeZero, videoAsset.duration), 
                                       of: videoAsset.tracks(withMediaType: AVMediaTypeVideo)[0], 
                                       at: atTimeM)  
        videoSize = videoTrack.naturalSize
    } catch let error as NSError {
        print("error: \(error)")
    }
    totalTime += videoAsset.duration // <-- Update the total time for all videos.

...