Let's say you have a TextEdit document, that contains clusters of passages:
[Allison]How are you, Jim?
[Fred]Fine, thanks.
...
The best you can hope for without considerable complexity, expense, or both is to fall back on the Automator application on OS X. You can create a very simple Automator Service that speaks selected text in the chosen voice. This Service can be tied to a unique keyboard shortcut, and each voice can have its own unique keyboard shortcut that provides a voice start/stop speaking capability for longer text passages.
Once the individual Automator Services for Allison and Fred voices are saved, the student can triple-click on a sentence or paragraph. I have embedded the [voices] at the beginning of the sentence, and added a feature to the Automator Service that tests the [voice]. Select the [Allison] sentence and use Fred's keyboard shortcut and nothing is spoken.
Automator is found in your /Applications folder. It is the robot with the pipe. When you first launch Automator, it will prompt you for a new document. Select new document and then double-click the Service gear. In the left Automator panel, are Libraries of actions. Select the Text category. Immediately to the right, will be a column of Text related actions. Locate, single-click, and drag the Filter Paragraphs action to the much larger workflow window and release it. Locate the Text action Speak Text, and drag/drop it below the Filter Paragraphs workflow item. Make certain that these are configured to reflect the following completed Allison Service workflow. You should now have the following (single-click to enlarge the image):
Save this via Automator > File > Save... and name it just Allison, or Allison-voice. From the Automator File menu, close the Allison Service workflow. Now, following the above instructions, repeat this process to make the Fred Service with Automator > File > New.
Copy the above TextEdit text into a new TextEdit document. Select the [Allison] line, and then see if the Allison Service is available under TextEdit > Services > Allison. If it is, select it and listen. If no Allison Service appears here — select from the Dock, System Preferences, and then the Keyboard preference. Select Shortcuts > Services and then scroll the right list of Services to find Text > Allison. It is probably selected, but deselect, and then reselect it. It should now appear under the TextEdit > Services menu. In the following Screenshot, you will see the Allison Service activated and assigned a control-A key to activate the voice on the selected text in your document. Repeat this process for the Fred voice Service, with a different keyboard shortcut than Allison.
It is a good idea to be aware of reserved keyboard shortcuts.
OS X
Pages ’09
Pages v5.
Voices
If you want to fine tune Allison's (or Fred's) speaking rate, you can do this in the Dictation & Speech > Text to Speech preference panel. If you check the Speak selected text when the key is pressed item, you can start and stop Allison speaking the selected document text in longer text passages. Since the default system voice uses Control+S, you probably want another short-cut specific to Allison. I chose Control+Q. Once Allison is speaking, press Control+Q to pause, and Control+Q to resume speech to the end, or until encountering [Fred]. When you configure Fred, you will need a separate Text to Speech shortcut for that voice.
In Summary, you now have two different voices that are initiated as Voice Services with two unique keyboard shortcuts. The individual voices can be paused or resumed once initial Voice Service is started. For a given voice, the student selects the voice specific text passage, and presses the associated keyboard shortcut for that voice. If the passage is a larger paragraph, the student may pause the voice with its unique pause/resume keyboard shortcut. If a student doesn't select a passage of text and presses the voice's pause/resume key, it will start speaking until it runs out of text. An undesirable side-effect.
You should have sufficient guidance above to create and activate two voices as you initially described.
