No that didn't worked I tryed everything, I used coolbits 18 value registry hack, to force the 2 card to use the same amount of memory and I have some problems with that, also I have postet to Nvidia Customer Care and This is what I got : I understand that you would like to use SLI configuration with GeForce GTS 250 graphics cards with different memory.
I am sorry to inform you that SLI will not work with different memory of graphics cards both the graphics card should be of same memory.
In order for SLI approval all 3 must match, 1) GPU architecture, 2) Device ID, 3) Frame Buffer Size. if any of these don't match then SLI will not work.
Enable coolbits option is EOL and we are going to remove this FAQ from the website.
So my only option now is to make the 1GB version card to use only 512mb or the video bios to boot it like that