When you say enable SLi on BIOS does it show 8-4-NC-8 or summit like that? Thats SLI enabled.
You should then be able to see it in the nvidia display settings and also see both cards in the control panel device manager. Turn the SLI Bridge (connected to both cards) the other way round if SLI is properly enabled in the BIOS - that should then give you some vision. If that does not work - try one card at a time and make sure they are working individually.
Make sure the MACH USB header is connected to the USB head and that you are connected to the row that just has 4 pins on it (not the row with 5 pins on it), if it is and you can't see it in windows then turn it around, the black cable on the mach plug should be on the 1st pin next to the spare - that being the case you have to re-install the mach drivers.