Little typo.... your BIOS is 2.38, not 2.58. Your issue fits under the universal adage "Nothing that money can't fix" and specifically nothing that Windows 7 Professional 64-bit can't fix.
Here is your answer: "Commercial servers, workstations, and other high-end PCs may have more than one physical processor. Windows 7 Professional, Enterprise, and Ultimate allow for two physical processors, providing the best performance on these computers. Windows 7 Starter, Home Basic, and Home Premium will recognize only one physical processor."
Microsoft source is HERE. Think of BIOS as a primative operating system..... it gets you into the early phase of booting up and thus BIOS can let you see the presence of things which your main OS may be programmed to not detect or work with (such as your second processor).
Buying a W7Pro64 bit System Builder license/DVD can give you an added benefit. From that OS you also can upgrade to W10Pro64 on that workstation.... there are still several ways to do that at no added cost. I have several xw6400 workstations working under W10Pro64, and even some as dual boot to W7/W10.
If you are interested..... my dual boot xw6400 workstations each have 2 SSDs.... There is a switch approach, but I prefer the IcyDock receiver/ 2 carriers approach. Works great, and the shutdown-boot up change from 1 OS to the other takes about 60 seconds. I use the MB991SK-B for that, plus 1 extra carrier for the second SSD. The receiver fits perfectly into the unused floppy bay, and a single SATA data cable goes from it down to SATA port 0 (blue plastic) on the motherboard. For a proven-to-work-great SSD: Intel 320 Series 300GB (or even 160GB) bought used off eBay, with use of the free Intel ToolBox for SSD management/firmware update if needed. Non-HP SATA3 drives can have issues with the HP SATA2 workstations like yours.