Blender 2.77 GPU SLI rendering problem

I have got two NVIDIA GeForce GTX 460 in SLI Configuration. I have always rendered images quite fast till 2.66b Blender version. Now with the new 2.77 version my rendering are extreeeeeeemly slow, also with the most easy and light image (cube + plane + sun + sky texture Preetham) so no memory problem. Much more fast with a single GPU or CPU
Thanks for any help.
Intel i7 950 - Windows 10 Pro - Nvidia Drivers 364.51 - Direct X 12

PS I had a similar problem some months ago which I solved upgrading the graphic drivers to the last version

You should not be using SLI for rendering with multiple gpus

Thank you very much. Have a nice day.