I was assuming that I could combine any two voices no matter how inappropriate that combination would be and that 'Big Brother Yamaha' was not going to 'guide me' to what they believe to be appropriate. As I said previously, I see the 'DUAL' display for some voices and I guess you're saying that at this point (I'll assume the volume default for the main voice here) that I can directly go to the 'SHIFT+DUAL' and grab my second voice. For these second voices I like to use the brass or woodwind category, but the trouble seems to start here, but I'll try that when I get home.....I'm at the tire shop now with a long wait.
For the most part, you can use whatever dual voice you want. There are just a few exceptions -- these "double voices" where Yamaha felt that, for the overall tone to have the best quality, it was necessary to use two tone generators -- the main voice and the dual voice -- to combine tones to make the overall sound. Kind of like using two oscillators on a synthesizer, instead of just one, to get a more complex and robust sound. On my keyboard, I think there are about 30 such sounds. You said that if you select one of these sounds, where "dual" is displayed, that you could just use shift+dual to grab your second voice. Yes, you can change the dual voice -- but remember, if "dual" came on automatically when you selected the main voice, then this is a voice that Yamaha set up to use both tone generators for the overall sound labeled in the main voice. If you then manually change the dual voice, then you will change the overall tone of what the main voice was meant to sound like, because you'll be changing half the tone.
Another example is piano/strings. At least on my keyboard, that is a "double voice", with the piano sound being the main voice, and the strings automatically set in the dual voice, with the dual voice mode automatically turned on. Yes, you can change the dual voice manually, but then you will no longer have the strings part of the piano/strings voice, and instead you'll have just a piano combined with whatever dual voice you select. Remember, this is only for main voices where the dual voice is turned on automatically. If you select any other voice where dual is not turned on automatically, then although Yamaha does put a "suggested" dual voice in the dual voice slot (without automatically turning the dual voice on), you can change this dual voice to mostly whatever you want, and it won't affect the main voice.
Also keep in mind that at least some of these "double voices", where Yamaha sets up main and dual voices for the overall tone, are represented in the XG Lite group of voices, and those (at least on my keyboard), are all regular single-tone voices that can freely be combined with a dual voice or act as a dual voice without losing any of the voice's original tone. An example is the piano/strings. My keyboard has a piano/strings in XG Lite that is just a regular single tone -- it does not turn on dual voice, and it can be fully layered with another tone. But the tradeoff is that the XG Lite piano/strings is not as full and vibrant of a sound as the main piano/strings that uses both the main and dual voices for the overall sound.