It just means you won't be able to use a second card. You will be able to use both cores on the 4870x2
Since the two GPUs are on the same PCB, it's a 'virtual' crossfire, with the crossfire control core being inside of the PCB itself, instead of on the motherboard as you would have with a dual card configuration.
Now, I'm not sure why you'd want a second 4870x2...