Cool thanks everyone...just the answers I was looking for. I assumed this would be fine as long as it was the same PCB but it is good to here many people have had this work...I am especially intrigued by Jdrom's running the 2 card at different speeds that's awesome.
I think it kills my Crossfire performance a bit though but I'd rather keep my HIS card (aka the main one) running fast so that in games that don't use Crossfire, they still run nicely.
However cadaveca brings up a good point about scaling and just overall support. I don't think it would be a worthwhile investment to get another HD4870X2. Probably see more gains in getting a new CPU or something instead.