Sergio, the tool just modifies your saved bios dump. You use nvflash to flash the BIOS.

Best thing to do is save your original BIOS using the dump function of GPU-Z & store a copy away somewhere safe.

Use KGB to adjust/edit another copy of your bios then use nvflash to flash it. I would untill you get used to it just flash one card at a time & test it before doing the other & running them together.

I would if you are not used to flashing or recovering a failed flash/card highly recommend putting in place an emergency recovery plan before doing anything. You can find how to do this all over the net but simply put you need to make a bootable disk/flash stick etc & have a good known working bios/stock one on there ready to go in an emergency. I personally fly close to the wind on stuff like this & just get on & do it as I am confident I can recover in some way. I have a saved stock bios & I can always recover a goofed card using a blind flash/second video card/onboard video or old school pci video card.