Don't be lazy!
If you are it will cause you problems with the drivers!
First remove BOTH your voodoo 2 cards from your mainboard.
1. Get Driver cleaner (older version NOT "Pro Version")
2. Run it and select "3Dfx" and click CLEAN...then wait.
3. Run CAB Cleaner from the "tools" menu - click CLEAN and wait longer. (close the program when it finishes)
4. Extract the FastVoodoo 2 drivers and shut down the PC.
5. Insert the first Voodoo 2 into your mainboard.
6. Boot your PC.
7. When Windows "Auto detects" the Voodoo 2 say NO to the auto install.
8. Run the FastVoodoo 2 installer - if you get any messages asking "do you wish to continue?" - then continue anyway and install.
9. Reboot the PC when asked.
10. Power OFF when you get back to the BIOS screen.
11. Insert the second Voodoo 2 and link them with the SLI cable. Then connect the passthrough cable from the SECOND Voodoo 2 (the second one you inserted) to the main GFX card.
12. Boot the system.
13. When Windows detects the second Voodoo 2 and asks to install the drivers again - continue to install.
14. If everything went well, you should now have SLI mode funtioning!
PHEW!
Come back and let us know how you get on...