2 4870 1GBs in CF should be faster than a 4870x2 IF you have at least 2 PCIe2.0 8x lanes.
So go for the CF 4870s.
No they shouldn't...
The clock speeds are the same, the x2 has 8x per card, which is the same, the graphics cores are the same. In basically every way, the 4870x2 is identical to a pair of 4870s in CF on 8x slots. On 16x slots (for both cards), the pair of 4870s would have more bandwidth, but I'd be surprised if it made a significant difference.
Honestly, for all practical purposes, they are identical in performance.