Mustaine & Poland were quite freaking excellent but so were Altus/Piercy. I dunno about the new Heathen since Ira Black is replacing Piercy. Ken Nardi and Kevin Heidbreder are a good team too. Too bad that three of my fave thrash bands (Voivod, Watchtower and Coroner) only have one guitarist so I can't mention them.
Also Rob Urinati and Joe Rico of Sacrifice, Jorgen Sandau and Michael Styzer of Artillery and Jim Durkin and Eric Meyer are excellent though they shine mostly just on the riff side and not on the leadside.