SLI - Originally it stands for Scan-Line Interleaving, but now the correction is Scalable Link Interface, which specifically refers to the direct connection between the two graphic card.

Not all PSUs are capable with powering two graphic card in SLI mode. You should look for the SLI-ready logo on the capable PSU package or web page , indicating the PSU is officially certified by Nvidia.

This page shows all certified SLI-ready PSUs: http://www.slizone.com/object/slizone_build_psu.html