You can do this by changing the Secondary Voice settings. To access these:

  1. Go to Options Options icon at the far right of the bottom toolbar.
  2. Tap the More info Icon Info@2X button next to the language name.
  3. Secondary Voice will probably be set to Same as Primary. Tap its setting.

If you want to use a completely different voice, tap Secondary Voice again. Then, tap another voice from the Installed Voices list to select it. If the voice you want is not downloaded, see Download or delete a voice.

You can also use the same voice as your Primary Voice, but change how it sounds. To do this, change the Relative Volume, Speech Rate, and/or Voice Personalization (pitch) settings.