In the AUDYA, the 2nd voice is not directly associated with the REGISTER but the lead voice. When you call up a lead voice, it (by default) has a 2nd voice. You can go through and for every lead voice, associate a 2nd voice with/to it. When done, simply SAVE the 2nd voice file (which remembers which 2nd voice to associate with which lead voice.
Now, when using REGISTRATIONS, you simply save the register with the lead voice you want to use (the associated 2nd voice is displayed below the lead voice). When you press the 2nd voice, it turns ON the 2nd voice you saved with it.
Hope this clarifies this.
Thanks,
AJ