I voted Marty/Jason coz Cacophony is so brilliant.
Next choices would've been KK/Glenn especially on the Painkiller album, all the Megadeth duos but especially Chris/Dave and Jeff/Dave coz the lead guitar attack on the first 3 'Deth albums is amazing, when Marty joined the band he did most soloing himself (RIP is an exception though..), and Steve/Phil coz I just love the way their guitar parts work together so perfectly especially on Hysteria, although Phil does 90% of the soloing himself.
Adrian and Dave also rock. Chris Holmes and Randy Piper weren't too great in my opinion (and Randy is way better than Chris), and the Slayer duo SUCK! I couldn't think of any other duelling guitar teams hehehe.
Do the guys from Queensryche do it?? I think they do, I don't know their names but theres some great lead guitar instrumental sections on Mindcrime!!!