I've never seen one for the e6600's Socket 775. You can for sure get a board that'll support two Socket 771 Xeons, the server versions of Core 2. But why would you want two e6600s anyway? A single q6600 would be better.
Not going to happen, its impossible infact. The entire Conroe family, including the 3xxx series of Xeons arent SMP capable. If you want a dual cpu setup with processors based on the same arch as the C2D you'll need a Woodcrest or Clovertown Xeon.
They have stackable cases, shelves, cubes, why not stackable processors?
Want to add power to your system? Place 1x e6600 over your current e6600 and double your performance. Still not enough, place 2x e6600 on top of your current one for a total of 3x processors in stack. Sounds good to me