在Android中合并两个视频文件可以使用MediaMuxer类和MediaExtractor类。以下是一个示例代码，演示了如何合并两个视频文件。

public class VideoMerger {
    private static final String OUTPUT_FILE = "/sdcard/merged_video.mp4";

    public static void mergeVideos(String videoFile1, String videoFile2) {
        try {
            MediaExtractor extractor1 = new MediaExtractor();
            extractor1.setDataSource(videoFile1);
            int trackCount1 = extractor1.getTrackCount();
            int videoTrackIndex1 = -1;
            for (int i = 0; i < trackCount1; i++) {
                MediaFormat format = extractor1.getTrackFormat(i);
                String mime = format.getString(MediaFormat.KEY_MIME);
                if (mime.startsWith("video/")) {
                    videoTrackIndex1 = i;
                    break;
                }
            }
            extractor1.selectTrack(videoTrackIndex1);

            MediaExtractor extractor2 = new MediaExtractor();
            extractor2.setDataSource(videoFile2);
            int trackCount2 = extractor2.getTrackCount();
            int videoTrackIndex2 = -1;
            for (int i = 0; i < trackCount2; i++) {
                MediaFormat format = extractor2.getTrackFormat(i);
                String mime = format.getString(MediaFormat.KEY_MIME);
                if (mime.startsWith("video/")) {
                    videoTrackIndex2 = i;
                    break;
                }
            }
            extractor2.selectTrack(videoTrackIndex2);

            MediaMuxer muxer = new MediaMuxer(OUTPUT_FILE, MediaMuxer.OutputFormat.MUXER_OUTPUT_MPEG_4);
            int videoTrackIndex = muxer.addTrack(extractor1.getTrackFormat(videoTrackIndex1));

            muxer.start();

            ByteBuffer buffer = ByteBuffer.allocate(1024 * 1024);

            MediaCodec.BufferInfo info = new MediaCodec.BufferInfo();

            while (true) {
                int sampleSize = extractor1.readSampleData(buffer, 0);
                if (sampleSize < 0) {
                    break;
                }
                info.presentationTimeUs = extractor1.getSampleTime();
                info.flags = extractor1.getSampleFlags();
                muxer.writeSampleData(videoTrackIndex, buffer, info);
                extractor1.advance();
            }

            while (true) {
                int sampleSize = extractor2.readSampleData(buffer, 0);
                if (sampleSize < 0) {
                    break;
                }
                info.presentationTimeUs = extractor2.getSampleTime();
                info.flags = extractor2.getSampleFlags();
                muxer.writeSampleData(videoTrackIndex, buffer, info);
                extractor2.advance();
            }

            muxer.stop();
            muxer.release();
            extractor1.release();
            extractor2.release();

            Log.i("VideoMerger", "Video merged successfully!");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

这段代码首先使用MediaExtractor类来提取视频文件的音视频轨道，然后使用MediaMuxer类来合并两个视频文件的视频轨道，并将结果输出到一个新的视频文件中。请确保在AndroidManifest.xml文件中添加了适当的权限，以便读取和写入外部存储。