Can 2 Ati or Nvidia GPU b used with sli/crossfire motherboard?

The only motherboards that have chipsets capable of SLI and Crossfire both are Intel P55 and X58 chipsets, to date anyway.
Why? Because previous to the release of these 2 chipsets, nVidia would not grant a license to anyone else allowing them to use their SLI technology (dumbasses).
If you wanted SLI you had to to buy a board with an nVidia based chipset. But unfortunately, nVidia chipset based motherboards, well they are stupidly expensive, and simply inferior in almost every way to their AMD/Intel based counterparts.
ATI/AMD on the other hand did allow Crossfire on Intel chipsets, as well as their own AMD chipsets. So, in effect, they made Crossfore much more attractive. The rest is history. nVidia is quickly sinking, I myself at one time preferred nVidia over ATI cards, but my next GPU upgrade will definetly be ATI. nVidia finally realized the error of their ways and decided to allow SLI to run on another platform besides their own, but it may be too little too late for them unless they get their act together, and fast.