why would you need dual 64bit cpu's.. it wont exactly make it a 128 bit cpu and run 4x faster.. you will not notice much increase compared to 1 64 bit cpu.
Cool, but why do you need 2 cpu's? Is this a gaming system you are trying to build? Unless you have some serious money to blow, I would spend that extra money that you're spending on that second Opteron on something else.