That's the problem though. Another member of this forum has the IDENTICAL cards as mine, in crossfire and his work great with Mantle. I get either loss of frames in comparison to Direct X or complete failure and crash to desktop. It really is dependant on the whole system config and as it seems, luck.
Desktop: FX8350 @4.7Ghz, Gigabyte 990FXA-UD7 Motherboard, 16GB Corsair Vengeance @1.7Ghz, Dual Asus R9 290X DCU2 @ 1.1Ghz, Kingston HyperX & V300 128GB SSDs, 4TB HDD, CM Inferno Gaming Mouse, Logitech G510, 3x Acer 223W Monitors@5040x1060, Antec DF-85, Corsair H100i, Corsair HX1050 PSU.
Laptop: MSI GX70: AMD A10-5750 @3.5Ghz, HD8970M @1.1Ghz, 16GB Corsair Vengeance @1.6Ghz, 250GB Kingston HyperX SSD, 1TB SSHD, 750GB Seagate, Gamdias Demeter Gaming Mouse, CM SF-17 Cooler.