Obviously AMD just want to better differentiate their newest product line up. They never used X900 since the x1900 series and I think it is a good call marketing wise to once again use it. Although the kind of people who buy high end dual gpu cards are at least somewhat informed ( face it a good portion go to high end pc boutiques which cater to wealthy , possibly ill informed buyers )
Nvidia did the whole GTX285/295 thing and I expect the 5870 and 5970 to be the same to the letter ( as in naming and hemlock being R870 at reduced clocks )

5970 is less confusing than 5870x2 if you ask me.