如何用just_audio合并两个音频文件同时播放？

import 'package:just_audio/just_audio.dart';

final player = AudioPlayer();

void loadAudioFiles() async {
  await player.setAudioSource(AudioSource.uri(Uri.parse('audio_file_1.mp3')));
  await player.setAudioSource(AudioSource.uri(Uri.parse('audio_file_2.mp3')));
}

final mixer = AudioMixer();
final audioSource1 = AudioSource.uri(Uri.parse('audio_file_1.mp3'));
final audioSource2 = AudioSource.uri(Uri.parse('audio_file_2.mp3'));

void mergeAudioFiles() {
  mixer.track(audioSource1);
  mixer.track(audioSource2);
  player.setAudioSource(mixer);
}

void playMergedAudio() {
  player.play();
}