@OP, here's how I would do it
Since you have 2 cards, it's 10 times easier that if you have only one because of one simple reason: if you make a bad flash, you can always boot from the second card and flash the one with the bad flash with the stock BIOS. With that in mind, here are a few steps:
- find the one with the lowest clocks; let's say you have the A card and the B card, and the B has the lower clocks. We'll call B the main card.
- make BIOS backups of both cards calling them Abackup and Bbackup; here's a great guide on OCN on how to flash BIOSes; you will be using a soft called nvflash to update the BIOSes
- now B is the main card due to the lower clocks; take the Bbackup BIOS, make a copy named Bnewbios, it will have the extension .rom like Bnewbios.rom
- very important - now is the fun part - shut down and remove the B card from the computer; place the A card on the uppermost PCIe slot; start and enter the BIOS, set the BIOS to boot from the uppermost PEG PCIe (called usually PCIe0 or PEG0) - check the manual if unsure. Restart.
- now you'll be using nvflash like in this guide: from the windows make/insert a bootable floppy, copy onto it the nvflash soft and the BIOS called Bnewbios.rom; restart and boot from the floppy
- use this command in the command prompt: nvflash -4 -5 -6 Bnewbios.rom
- now the BIOS from the B card is updated on the A card - two things can happen:
- all is ok, you'll have 2 B cards
- the A card with B bios won't boot; then you'll have to make a blind flash; you can shutdown, remove the A card, insert the B card, boot in Windows and put on the floppy the Abackup.rom BIOS - next you'll swap back in the A card with the bad flash, and make a blind flash with the new BIOS: wait for it to boot from the floppy and after it doesn't make any noise type nvflash -4 -5 -6 abackup.rom
In case all went fine with the flash, you have 2 identical cards to put in SLI; if it didn't you'll be back to square 1 with an A and a B card; in the latter case you can search on the web for BIOSes that are likely to work on both cards, thus you'll have a Cnewbios.rom to flash on both of them.
Hope something works as they're 2 identical cards only with different names, the chance of a bad flash is real slim.