Text to Speech TTS Software for Mac

I was looking for a text to speech (TTS) Software for Mac that had the ability to output a audio file switching back and forth between several different voices as if to hear a conversation between two characters. For example, if I transcribed a screenplay, I could be able to hear the dialogue between two characters in two different voices.

Does anyone know of a TTS Software for Mac with this ability?

Preferably free or with a free trial/download?

If I'm not mistaken, OS X has text to speech built right in. For example, if you have TextEdit, you can highlight the text you want to be read, go to the TextEdit menu => Services => Speech => Start Speaking Text. This is a universal service, so it works the same way in just about any program. You can set different voices in System Preferences => Speech => Text to Speech tab.