Some more info from Hardware Canucks:
"In order to properly set the stage here, it is important to discuss why AMD seems to have failed in their efforts to create a bona fide “official” dual GPU product in spite of an initial lead over NVIDIA. The issue here isn’t a lack of intent, strategy or focus but rather technological limitations that caused some insurmountable roadblocks during development. Despite being fabricated on a relatively efficient manufacturing process, AMD’s Tahiti cores tend to run quite hot and require a significant amount of power. Careful binning, judicious application of voltage constraints and lower clock speeds could have allowed a dual HD 7970 card to come to fruition but the competition’s Kepler architecture prevented that. The quick release of NVIDIA’s GTX 690 delivered a crushing blow to AMD’s exclusive claim over the performance crown and shattered the performance per watt ratio for dual GPU products. Competing against it wasn’t impossible but even after numerous delays the effort would have required a massive investment for a limited return. We’re guessing the ROI ratio didn’t sit quite well since those resources could have been –and will likely be- put towards upcoming architectural developments instead.
Naturally, some still held hope for the mythical HD 7990 and everyone wanted to avoid a GPU generation without a halo Radeon product. From what we have heard, AMD finally dropped the project a few months ago and handed off the hot potato to board partners, basically giving out the baseline specs and naming rights while deferring the financial burden. Given the market conditions, there weren’t many takers but PowerColor did step up to the plate and the end result is their HD 7990 Devil 13."
http://www.hardwarecanucks.com/forum/hardware-canucks-reviews/56819-hd-7990-review-powercolor-s-devil-13-a.html