Your Favorite Twin Guitar Duos?

Tipton/Downing - Priest (a given, at this point)
Murray/Smith/Gers - Maiden
Schaffer/Santolla - Iced Earth
Hetfield/Hamett - Metallica
Oliva/Caffery - Savatage
Mustain/Friedman - Megadeth
Loomis/O'Brien - Nevermore
Linsk/Tailor - Overkill
Grapow/Weikath - Helloween
Pyres/Antsis - Cradle of Filth
Schuldner/Koelble or Schuldner/LaRocque - Death

That's all I can think of at the moment.
 
As to Slayer not doing harmonies you are seriously wrong. The middle section to Angel of Death has them, their signature tune. You have missed it all these years. They at times harmonise in flat 5ths (the devils interval). Man it is there in the breakdowns all the time. On many many songs.

The H-Team one of my favourites.

Carcass dudes rock. What a combination.

Dark Angel- Time Does Not Heal amazing performance.

The Young brothers. Malcom Young is amazing in his straight ahead yet unmistakable rhythm guitar

Chis Poland and Mustaine on Peace Sells.

Thin Lizzy

Maiden

Queensrhyche

Midnight Oil (Early stuff before they became an enviromental fascist band)

Sepultura- Max era

Cynic

Suffocation