Just a few quick things, do you have the EZ 12v molex plug in the mother board ? Have you changed over the SLi card in between the PCI-E slots ?
Have you got the latest drivers from
nvidia
If so go into the BIOS then go to Advanced or some where like that and there shold be a SLi option [AUTO][Normal][SLI] and change it to SLI then save and exit. You must have the EZ 12v power in to do this. If that don`t you it does sound like you have a BIOS issue with the cards!