Check your Scanline Interleave status, and check to see if your frame buffer memory and texture memory is being detected right. Then of course compare benchmarks.

Here's how my Voodoo 2 12MB SLI setup looks:

The best way to check is running some game or even 3Dmark at 1024x768 resolution. If that works, then your SLI is ok.

Thanks a lot - everything is fine.
The driver says "SLI: detected" and 3DMark runs fine on Voodoo & 1024x768 (=> 3460 3DMarks).
All I need now are some suitable games to run on this machine 😉

Yeah, that check in the control panel, 1024x768, and running benchmarks is pretty much all you need. I generally throw Deus Ex at it as a practical test, as an example of a late life Glide game that will tax pretty much any 3dfx hardware you throw at it.