Not all the X58 boards are SLi capable, if yours were then you should have got an SLi bridge bundled with the documents as they are usually supplied with the motherboard whereas Crossfire bridges tend to be supplied with the graphic cards.
I have the same problem and set up as ONT, except I do have an upgraded bios. But, I am not sure if it is a mobo problem on mine, I have a P2370 Samsung Syncmaster monitor, and the problem I am having is the monitor will not come on at all, dead screen with a blinking power light. It works fine long as I have it plugged in to either GTX 470 that is slotted in the main slot but soon as i plug in the second GTX none of the monitor ports will fire the monitor. Both gpus have power and seem to be working but, nothing. It seems, as I have read many articles on sli'ing this mobo, that most people at least get to the nvidia UI and find that sli is not working. Any help or research on this will be greatly appreciated.