This will be easier if you have a way of recording yourself, or if you have a buddy that can play one of the parts. If you need a cheap recording solution, there's a recording program that I used to use called WavePad, and its not the best, but maybe they've improved it. You can find it
Anyways, duo harmony is exactly what it sounds like - its simply two or more guitar parts that harmonize with each other, and this works best for lead playing. A lot of flashy heavy metal players are using this trick extensively today, and I have to admit - its quite addictive.
Let's take a look at several ways that we can do this. The simplest method is to take or write one set of notes, and then play the exact same notes, exactly one octave apart (either higher or lower - its your choice).
Here's an example, when put together they have an awesome effect.
Note for note, those passages are the same, but just one octave apart. Something that you hear a lot of in some of the more popular metal bands right now is another type of duo harmony. This is the same harmony that is common in Cuban piano work. In metal guitar, the sound of two lead parts played in this particular way presents a
sour, wicked result.
The formula for this approach is simple. Take a lead part, then when you go to harmonize with it, instead of playing it one octave apart, drop the starting note down 3 notes. That's a whole plus a half step, W+H.
Here it is in action:
And yet one more that I know about is below. The formula for this one is to take your first lead passage, and when you to the next one, drop the starting note (from the octave position) down 5 notes.
There are so many different ways that you can construct these harmonies, and the best way to get this locked into your head, is to do a little bit of experimentation with each method, and even better - see what you can come up with.
The key is really to start with the first notes of both passages. Come up with something, and then try to find the first note of your lick in a different area, exactly one octave higher or lower. After you get to that spot, you can then start to move the overlapping lick around to see what works.
Best of luck - I hope I explained this well and that this helps you guys out.