The nVidia driver looks to make sure the board is on the approved SLI list. If it is not, then SLI is not allowed. There is some check that is added in the BIOS and that is what the driver checks for.
Like I said, HP may have paid for SLI on Quadro cards, but that doesn't mean that the same fee pays for Geforce cards. Though that is just speculation on my part.
Your best bet would be to contact HP and ask them is SLI with GeFroce cards are supported on this system.