As Yups says, Intel Motherboards/Graphics do not support Dual DVI, so there is no problem.
I can't send you graphic reports, because I now use a graphic card, a radeon 7750 club 3d, and now I can get the right solution, 2560*1600.
But I can confirm, that the problem was the same with a nother Dual DVI-D cable. No video adapters. We updated the Bios today, so I do not know which bios we used that day.