Doesn't sound normal.

Is vsync enabled or disabled?
Are the cards the same make and model?
Is interlacing happening in all games?
What driver version are you using?
What refresh rate are you running at?

SLI works by interleaving scanlines so it is similar to interlacing but it should not typically be visible.

Turning off the don't sync buffers in the OC tab helps a lot. However with explosions, plasma balls you still see the interleaving clearly.
Both cards are a Skywell Magic 3D II, however one is rev 3, other 3A. The cards are from '98 and '99.
Problem is now gone in both Q3 and HL
Fastvoodoo 4.6 (98SE) and 4.0(XP)
60Hz

Sidenote 1: In XP, Voodoo apps don't return to the deskop
2: The PSU makes farting noises when using the Voodoos, seems they are quite the power draw

It sounds to me as if there is insufficient power going to the video cards which is causing them to lag out of sync. Of course, this is a guess and I could be completely wrong. It does sound like your PSU is unsatisfactory though.

If VSync is off you will see the Cards current image render status in the common framebuffer. This looks like interlaced on fast movements if one card takes longer for the even scanlines than the other for the odd scanlines. This is completely normal, the simple fix is to turn VSync On.

You can of course enhance this effect if you downclock one card and overclock the other.

About power draw, even a Voodoo5/Pentium III system draws only 90W in full load on a newer PSU (a bit more on older PSUs obviously). If your PSU can't even handle this without causing noises I don't see any point in continuing to use it.