Favorite Duo/trio/etc that has been in 2 or more bands together
 

mine are :
Rick Froberg/John Reis (Drive Like Jehu,Pitchfork,Hot Snakes)
All the guys that were in Slint,Rodan,June of 44,Squirrel Bait
Omar R/Cedric B. (ATDI,Mars Volta,De Facto)
Andy Granelli/Tim Pressley (Nerve Agents,Model American)
Josh Homme/that guy that got kicked out (QOTSA, Kyuss)
Don Devore/Joe Cardamone (Souls She Said/Icarus Line)
all the guys from Rage (Rage, Audioslave)

Bill Bruford and Tony Levin - Both played with Yes and King Crimson.

Flea and Dave Navarro - Both played with Red Hot Chili Peppers and Janes Addiction.

Slash and Duff - Both played in Guns N Roses and Velvet revolver.

Jonathon Mobley and Ben Lord both in Falling Forward and Elliott
Tomas Kalnoky, Josh Ansley, and Jamie Egan in Streetlight manifesto and Catch 22
also the already mentioned ratm, audioslave and qotsa, kyuss

Mike Watt/George Hurley (Minutemen and fIREHOSE)

Chris Frantz and Tina Weymouth - Talking Heads and Tom Tom Club.

The guys of Rage and Audioslave.
I'm not sure if this countes but Dave Grohl of the Foo Fighters and Nirvana.

And I know somebody mentioned MinuteMen and that made me think of Mike Watt. He did just about eveyrthing and most of what he touched rocked.
Just thought I'd say that.

Uh . .
Peace

Brett Anderson / Bernard Butler - Suede/The Tears

Peter Hook/Bernard Sumner/Stephen Morris - Joy Division/New Order

Frank Black / Joey Santiago - The Pixies /Frank Black & The Catholics

Slash and Duff - Both played in Guns N Roses and Velvet revolver.

Same thing for me

Cedric/Omar - ATDI/TMV
Mike Mcready/Stone Gossard/Jeff Ament - Pearl Jam/Temple of the Dog/Mother Love Bone

Matt Freeman and Tim armstrong - both played in 'Operation Ivy' and 'Rancid'..
two awesome bands...
Billie.J.Armstrong and Mike Dirnt - Both played in 'Sweet Children' and 'Green Day'..