When doing an SLI or Xfire, you must get the exact same card. Hopefully you can exchange the new card for a GTX 460 SE (may be a price difference). Otherwise you are stuck with two graphics cards and no performance gain for it.
GTX 460 and GTX 460 SE are not same CUDA cores.
sli is possible when different brand and same model if there is any change in model you shouldn't able to sli .