Schuldiner used a lot of 5th harmonies - almost exclusively in fact. One thing he didn't do a lot was vary the harmony - my only gripe about Death in general (excellent band IMO). He nearly always played straight 5th's as opposed to harmonised counterparts that went off in a different directions - but that is what created that instantly recognisable style.
Play the route note then go to the next string and move up two frets (just like a power chord) - that's basically your harmony. He used to base most of his work on the harmonic minor scale - gives that dark eastern feel.
The reason this harmony works well is that it forms a powerchord - but each note is played cleanly and clearly on separate guitars as opposed to two guitars playing powerchords. This work very well for intricate passages especially if you want to make them sound dark and heavy - sometimes the bass will play an octave lower than the route note or an octave hgher to create a chord structure. Hope that helps.