Yes, you can use the voices from Ivona and AT&T for the same file in Text Speaker. First, install the voices you want to use. Once you have installed the voices, they will be listed in Text Speaker. Here is a list of available voices: https://www.deskshare.com/text-to-speech-voices.aspx
Now that the voices are working in Text Speaker, select the text you would like in the other voice, right click and choose the ‘Insert Voice Change’ option and then choose a voice. Click on Play to test your changes.
Learn more about Changing the voice.