No that's not true. A single HD 6950 is capable of driving up to 4 monitors, read today on Tom's news that they released drivers for 5 monitors but still need to check out.
Single GTX 570 is only able to run 2 monitors, 3 monitors with 2 of it in SLI mode called nVida 2D/3D surround.
Besides, you can't use to different GPUs, nVidia is a thing and AMD is another thing.
AMD has CF and nVidia has SLI (Multi GPU technologies to enhance the 3D and gaming performance )
If you want to use 6 monitors setup get a single HD 6990 or 5870 Eyefinity edition, both support up to 6 monitors.