How can I get dual track editing working with AVFoundation?

Question

I am trying to build a video mash up app and I need the user to be able to set up 1 track with a constant video track, and have a second track that will act as a b-roll track where it will cut away from the main track now and then to show some relevent content. I have the first track going so the clips on the timeline get arranged together in the composition using AVMutableCompositionInstructions but I can't wrap my head around how I can then work in the independently controlled b-roll track? I have been struggling with this for days! Here is the code that builds the first track worth of content, and I have it set now to dip to black between clips. Any AVFoundation guru's out there that can give me a hint?

CMTime nextClipStartTime = kCMTimeZero;
    NSInteger i;
    CMTime transitionDuration = CMTimeMakeWithSeconds(1,30);
    AVMutableCompositionTrack *compositionVideoTrack[2];
    AVMutableCompositionTrack *compositionAudioTrack[2];
    compositionVideoTrack[0] = [composition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
    compositionAudioTrack[0] = [composition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
    compositionAudioTrack[1] = [composition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
    compositionVideoTrack[1] = [composition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
    AVMutableCompositionTrack *bedMusicTrack = [composition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
    i = 0;
    NSMutableArray *allAudioParams = [NSMutableArray array];
    AVMutableAudioMixInputParameters *audioInputParams[2];
    audioInputParams[0] = [AVMutableAudioMixInputParameters audioMixInputParameters];
    audioInputParams[1] = [AVMutableAudioMixInputParameters audioMixInputParameters];
    [audioInputParams[0] setTrackID: compositionAudioTrack[0].trackID];
    [audioInputParams[1] setTrackID: compositionAudioTrack[1].trackID];
    float lastVol = 0;
    NSMutableArray *instructions = [NSMutableArray array];
    for(ClipInfo *info in videoLine.items){
        AVAsset *asset = [AVAsset assetWithURL:info.url];
        CMTimeRange timeRangeInAsset = CMTimeRangeMake(info.inTime, info.duration);
        AVAssetTrack *clipVideoTrack = [[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];

        [compositionVideoTrack[0] insertTimeRange:timeRangeInAsset ofTrack:clipVideoTrack atTime:nextClipStartTime error:nil];
        AVAssetTrack *clipAudioTrack = [[asset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];
        [compositionAudioTrack[0] insertTimeRange:timeRangeInAsset ofTrack:clipAudioTrack atTime:nextClipStartTime error:nil];
        if(i != 0){
            [audioInputParams[0] setVolume:lastVol atTime: CMTimeSubtract(nextClipStartTime,CMTimeMakeWithSeconds(1,30))];;
        }
        [audioInputParams[0] setVolume:info.volume atTime:nextClipStartTime];
        lastVol = info.volume;

        CMTime clipStartTime = (i == 0) ? nextClipStartTime : CMTimeAdd(nextClipStartTime,transitionDuration);
        CMTime clipDuration = (i == 0 || i == (videoLine.items.count - 1)) ? CMTimeSubtract(timeRangeInAsset.duration, transitionDuration) : CMTimeSubtract(timeRangeInAsset.duration, CMTimeMultiply(transitionDuration, 2));
        if([videoLine.items count] == 1){
            clipDuration = timeRangeInAsset.duration;
        }
        if(i != 0){
            //trans in
            AVMutableVideoCompositionInstruction *inInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
            inInstruction.timeRange = CMTimeRangeMake(nextClipStartTime, transitionDuration);
            AVMutableVideoCompositionLayerInstruction *fadeIn = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:compositionVideoTrack[0]];
            [fadeIn setOpacityRampFromStartOpacity:0 toEndOpacity:1 timeRange:CMTimeRangeMake(nextClipStartTime, transitionDuration)];
            inInstruction.layerInstructions = [NSArray arrayWithObject:fadeIn];
            [instructions addObject:inInstruction];
        }

        AVMutableVideoCompositionInstruction *passThroughInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
        passThroughInstruction.timeRange = CMTimeRangeMake(clipStartTime,clipDuration);
        AVMutableVideoCompositionLayerInstruction *passThroughLayer = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:compositionVideoTrack[0]];
        passThroughInstruction.layerInstructions = [NSArray arrayWithObject:passThroughLayer];
        [instructions addObject:passThroughInstruction];

        if(i < (videoLine.items.count - 1)){
            //fade out
            AVMutableVideoCompositionInstruction *outInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
            outInstruction.timeRange = CMTimeRangeMake(CMTimeAdd(clipStartTime,clipDuration), transitionDuration);
            AVMutableVideoCompositionLayerInstruction *fadeOut = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:compositionVideoTrack[0]];
            [fadeOut setOpacityRampFromStartOpacity:1.0 toEndOpacity:0 timeRange:CMTimeRangeMake(CMTimeAdd(clipStartTime,clipDuration), transitionDuration)];
            outInstruction.layerInstructions = [NSArray arrayWithObject:fadeOut];
            [instructions addObject:outInstruction];
        }
        nextClipStartTime = CMTimeAdd(nextClipStartTime,timeRangeInAsset.duration);
        if(i == ([videoLine.items count] - 1)){
            [audioInputParams[0] setVolume:info.volume atTime:nextClipStartTime];
        }
        i++;
    }

Answer 1

You need to toggle back and forth between video/audio composition tracks "A" and "B", or in your code compositionVideoTrack[0] and compositionVideoTrack[1]. Each iteration through the loop switch out which track you are compositing to.