Update. I've received news from nVidia support:
"We now have a clear response from HP. There is a problem with the BIOS of the DL580 which results in the GPUs in the S2050 chassis not being recognised. Also, the bandwidth between the DL580 and the GPUs is not as high as it should be; the bandwidth problem has been observed when testing an earlier NVIDIA product (S1070) with the DL580. HP is working on the BIOS problem, and also HP and NVIDIA are cooperating to resolve the bandwidth problem."
So, I'm waiting... Thanks.