Android MediaPlayer的同步启动可以通过使用CountDownLatch来实现。CountDownLatch是一种同步工具类，它可以使一个线程等待其他线程完成一定的操作后再执行。在这种情况下，我们可以使用CountDownLatch来使主线程等待两个MediaPlayer都准备完成后再启动它们。下面是代码示例：

public class MainActivity extends AppCompatActivity {
    private MediaPlayer mediaPlayer1;
    private MediaPlayer mediaPlayer2;
    private CountDownLatch latch = new CountDownLatch(2);

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mediaPlayer1 = MediaPlayer.create(this, R.raw.audio1);
        mediaPlayer2 = MediaPlayer.create(this, R.raw.audio2);

        mediaPlayer1.setOnPreparedListener(new MediaPlayer.OnPreparedListener() {
            @Override
            public void onPrepared(MediaPlayer mp) {
                latch.countDown();
            }
        });

        mediaPlayer2.setOnPreparedListener(new MediaPlayer.OnPreparedListener() {
            @Override
            public void onPrepared(MediaPlayer mp) {
                latch.countDown();
            }
        });

        try {
            latch.await(); // 等待两个MediaPlayer都准备好
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        mediaPlayer1.start();
        mediaPlayer2.start();
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        if (mediaPlayer1 != null) {
            mediaPlayer1.release();
            mediaPlayer1 = null;
        }

        if (mediaPlayer2 != null) {
            mediaPlayer2.release();
            mediaPlayer2 = null;
        }
    }
}