You can add multiple subtitles using the addSubtitles method provided by the `protonemedia/laravel-ffmpeg` package. This method accepts an array of subtitle files, where each entry in the array is an instance of the `FFMpeg\Coordinate\Subtitle` class.`

    use FFMpeg\Coordinate\Subtitle;
    use Protonemedia\LaravelFFMpeg\Support\FFMpeg;
    
   $format = new FFMpeg\Format\Video\X264();

    //change this will your path
    $saveVideoPath = 'path/to/output.mp4';
    $ffmpeg = FFMpeg::create();
    
    $video = $ffmpeg->open($videoUrl);
    
    $subtitles = [
        new Subtitle('path/to/subtitle1.srt'),
        new Subtitle('path/to/subtitle2.srt'),
        new Subtitle('path/to/subtitle3.srt')
    ];
    
    $video->addSubtitles($subtitles);
    
    $video->save($format, $saveVideoPath);

//May be it will help you...