In regards to gaming...
NVIDIA uses a hardware based buffer to eliminate microstutter from SLI configurations. They have had this in place since the 8xx0 series Geforce cards. This is the reason that SLI feels so smooth when you're using them in SLI, TRI-SLI, and Quad-SLI configurations.
AMD allows the cards to go balls to the walls performance, and doesn't have any hardware to smooth out the random pauses that happen with texture loading, vertex loading, scene rendering, etc. that can add a hiccup in the pipeline, which is what microstuttering is. The Radeon Pro mod adds a software buffer to put the brakes on the cards and gives them a split second interval to get this done, and DRASTICALLY improves Crossfire performance. On an aside, TRI-Fire doesn't have a microstutter issue, or a drastically reduced microstutter issue, but crops up again in QUAD-Fire (along with abysmal QUAD-Fire performance.)
Also... dual card systems I've had and played on personally
8800 GTX 320 SLI (owned)
4850 Crossfire (owned)
5830 Crossfire (owned)
5850 Crossfire (owned)
GTX 480 TRI-SLI (played on)
6870 Crossfire (owned, sucked)
GTX 590 (Dual CHIP CARD SLI, played on)
7970 Crossfire (owned, F***ING awesome)
And Dual chip cards 5970, 6990, 7970x2(Powercolor, HIS), 9800GT2, GTX295, GTX590, GTX690 are all Crossfire/SLI cards on a single PCB.
What means owned? You used those corosfire solutions for office aplications?