I don't know too much about SLI, but I think that when you set two or more cards up the clock speeds must be in sync. If one card can't handle 850MHz but the other can, the SLI won't accept 850MHz. You'll have to accept lower.
And regarding the temperature, I assume that 80C is fine. The maximum operating temperature of a 460 is 104C
, and having two cards with an OC will raise the maximum temperature above a single OC card.