Over the years, several metal artists have got together and sang duets or just cameo'd on each others records. (I'm not talking about Tribute Albums because those are a dime a dozen and those albums don't 'belong' to the artists).
Here are some of examples:
Alice Cooper & Jon Bon Jovi: Trash
Guns n Roses & Alice Cooper: Under My Wheels
Edguy & Hansi Kursh (Blind Guardian): Out of Control
Anthrax & Public Enemy: Bring The Noise
Phil Lynott & Gary Moore: Parisianne Walkways
Here are some of examples:
Alice Cooper & Jon Bon Jovi: Trash
Guns n Roses & Alice Cooper: Under My Wheels
Edguy & Hansi Kursh (Blind Guardian): Out of Control
Anthrax & Public Enemy: Bring The Noise
Phil Lynott & Gary Moore: Parisianne Walkways