Are they 12mb cards? What if you swap the cards around, same result?

If each card on its own tests ok, I would be blaming the SLI cable.
Swap the cards around as sugested above as thats a cheap easy fix, if that doesnt work try making another SLI cable, It's really easy just need a floppy cable.
https://www.vogonswiki.com/index.php/How_to_m … doo_2_SLI_cable

Not that is should matter but what driver are you using?

Now, in 1024x768, there's no more image, black screen and I have to hard reboot.

You are saying "now". Are the cards still both working correctly when using each single?

I had a similar issue with my SLI setup. Both cards ran fine individually. They even ran fine together in 1024x768 but in 800x600 the image had a reddish hue and in 640x480 they would crash.

It turned out one of the chips had a bent leg that was shorting out.

No idea why it crapped out in different ways in different resolutions and only in SLI, but it did. I fixed the leg and it works flawlessly since.

Check your card for bent chip legs.

I would second that recommendation. To date, I have bought a Diamond Monster 3D 4MB, a Diamond Monster 3D II 8MB, and two STB V2-1000 12MB cards.

The Diamond Monster 3D II 8MB Voodoo2 card wasn't working properly when I first got it off of Ebay, but it turns out it was the edge pins on the top of the center chip that was bent over and touching its neighbor. A strong knife edge and some VERY steady hands later and those pins are now roughly strait and not touching, then I stuck the card on a shelf for about 4 months. I tried sticking it into a computer so I could record the results of the glitches on-screen and found out there were no glitches anymore - the card worked exactly as it should.

The STB V2-1000 12MB cards are identical, yet wouldn't do SLI with the drivers on the included CD-ROM which was the original driver disc that came with the cards in the first place, nor with the official Voodoo2 drivers from the web. It wasn't until the nice folks HERE on Vogons pointed out the FastVoodoo2 drivers that I got SLI to work properly - so now I can game in glorious 1024x768 GLIDE. This will be very helpful indeed, as I plan to start streaming retro games on Twitch on my channel.

If anyone is actually interested, my Twitch name is "BeardedRetroGuy".