要在主层和叠加层上使用相同的视频源，可以使用AVMutableComposition来合并两个视频轨道。

以下是一个使用Swift的示例代码：

import AVFoundation

// 创建主层视频轨道
let mainComposition = AVMutableComposition()
guard let mainVideoTrack = mainComposition.addMutableTrack(withMediaType: .video, preferredTrackID: kCMPersistentTrackID_Invalid) else {
    fatalError("Failed to create main video track")
}

// 创建叠加层视频轨道
let overlayComposition = AVMutableComposition()
guard let overlayVideoTrack = overlayComposition.addMutableTrack(withMediaType: .video, preferredTrackID: kCMPersistentTrackID_Invalid) else {
    fatalError("Failed to create overlay video track")
}

// 获取视频资源
guard let videoURL = Bundle.main.url(forResource: "main_video", withExtension: "mp4") else {
    fatalError("Failed to load main video")
}
let videoAsset = AVURLAsset(url: videoURL)

// 将视频资源添加到主层视频轨道
do {
    try mainVideoTrack.insertTimeRange(CMTimeRange(start: .zero, duration: videoAsset.duration), of: videoAsset.tracks(withMediaType: .video)[0], at: .zero)
} catch {
    fatalError("Failed to insert main video track")
}

// 将视频资源添加到叠加层视频轨道
do {
    try overlayVideoTrack.insertTimeRange(CMTimeRange(start: .zero, duration: videoAsset.duration), of: videoAsset.tracks(withMediaType: .video)[0], at: .zero)
} catch {
    fatalError("Failed to insert overlay video track")
}

// 创建视频合成指令
let mainInstruction = AVMutableVideoCompositionInstruction()
mainInstruction.timeRange = CMTimeRange(start: .zero, duration: videoAsset.duration)

// 创建主层视频合成层
let mainLayerInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: mainVideoTrack)
mainInstruction.layerInstructions = [mainLayerInstruction]

// 创建叠加层视频合成层
let overlayLayerInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: overlayVideoTrack)
overlayLayerInstruction.setTransform(CGAffineTransform(scaleX: 0.5, y: 0.5), at: .zero)
overlayLayerInstruction.setOpacity(0.5, at: .zero)
mainInstruction.layerInstructions?.append(overlayLayerInstruction)

// 创建视频合成
let videoComposition = AVMutableVideoComposition()
videoComposition.instructions = [mainInstruction]
videoComposition.frameDuration = CMTime(value: 1, timescale: 30)
videoComposition.renderSize = CGSize(width: mainVideoTrack.naturalSize.width, height: mainVideoTrack.naturalSize.height)

// 导出合成后的视频
guard let exportURL = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first?.appendingPathComponent("output.mp4") else {
    fatalError("Failed to create export URL")
}
let exporter = AVAssetExportSession(asset: mainComposition, presetName: AVAssetExportPresetHighestQuality)
exporter?.outputURL = exportURL
exporter?.outputFileType = .mp4
exporter?.videoComposition = videoComposition

exporter?.exportAsynchronously(completionHandler: {
    switch exporter?.status {
    case .completed:
        print("Export completed")
    case .failed:
        print("Export failed: \(exporter?.error)")
    case .cancelled:
        print("Export cancelled")
    default:
        break
    }
})

这个示例代码假设你有一个名为"main_video.mp4"的视频资源文件。它创建了一个主层和一个叠加层的视频轨道，并将它们添加到AVMutableComposition中。然后，它创建了一个AVMutableVideoComposition来控制视频的合成和叠加层效果。最后，它使用AVAssetExportSession将合成后的视频导出到指定的输出URL。