I have done a bit more research and I will have to discount the SlashDot review.
NVIDIA does have a list of HP PCs that are SLI capable. They are, as another member stated, only SLI-certified with Quadro video cards. The older Z400 workstation is not one of them. The more recent HP Z440 workstation is rated SLI Certified by NVIDIA. See the following current NVIDIA web document.
http://www.nvidia.com/object/sli-certified-systems-and-motherboards.html