how do you make 2 guitars work together with octave chords?

[a e i o u]

I've been trying to get the guitarists in my punk hardcore kind of band to do this but no one gets how. Alot of times in these melodic fast and heavey bands one guitar plays power chords but then the other guitar plays and octave power chord over and and does some kind of melody. this happens in punky style music too. how do you do it? i heard the 2nd guitar should play a 5th or a 3rd above what the other guitar is playing. an example of this that many of you will be able to relate to would be on Green Day's song Bolevard of Broken Dreams, and American Idiot. In the chorus of the song, the guitars seem to be doing this. How can I make my own riffs like this?? I know this isn't bass related but i figure you guys know a ton about this stuff. thanks alot