Maybe because of the filter from 1 of the 2d cards, one can be a little better signal as the other.

For that reason lots of people liked to use matrox cards as their 2d choice as it has the best signal quality. Sometimes a better quality pass-trough cable works better as well.

The interference is some kind of shimmering distortion that distorts anything. For example any in windows any window borders become kind wavy when they are in the distortion.
The cards are the ones I tested here My 3DMark01 Mega Thread
Image quality in both looks just fine when they are directly connected to the monitor and Geforce 2 MX continues to look fine even when the signal goes through the voodoo 2 card, but the other one doesn't
I don't know if it is some kind of sync issue but the signal shouldn't degrade that much when going through vooodoo 2 when picture quality is just fine otherwise, right?

Is this distortion only in horizontal direction ? Chances are HSYNC isn't connecting very well and this will muck up any ADC level calibration too, leading to discolorations on those lines.

Tiido wrote:

Is this distortion only in horizontal direction ? Chances are HSYNC isn't connecting very well and this will muck up any ADC level calibration too, leading to discolorations on those lines.

Not a very good one is a picture that shows some of the distortion:

It looks even stronger in real life because of the shimmering it causes.

Is there any way to make the card work with the voodoo 2 card?

This is definitely horizontal only distortion. Outside of ruling out HSYNC connection issue between the cards, I can only suggest trying a different monitor.

Tiido wrote:

This is definitely horizontal only distortion. Outside of ruling out HSYNC connection issue between the cards, I can only suggest trying a different monitor.

Why the issue is with just one card then? Everything works fine with geforce 2 mx and same passthrough cable and same monitor. Same also works fine when I connect this same monitor to geforce mx4000 directly without going through the voodoo 2 card.
Can hsync signal degrade so much going through voodoo 2 card that if the signal would not be as good with one card originally that it would cause problem like this?

I don't know the details of how the card performs its magic but from monitor's perspective, such jiggle can only be explained by edges of the HSYNC signal getting rounded out to an extent that it is no longer possible to reliably detect start or end of a line. Since all else appears to work the problem is confined to single area in the electronics.

Tiido wrote:

I don't know the details of how the card performs its magic but from monitor's perspective, such jiggle can only be explained by edges of the HSYNC signal getting rounded out to an extent that it is no longer possible to reliably detect start or end of a line. Since all else appears to work the problem is confined to single area in the electronics.

Mystery deepens. I switched refresh rate from optimal to 60hz in windows and the distortion disappeared.
The distortion is still in dos though because dos uses 70Hz mode by default.
Based on the menu in the monitor "optimal" means 85Hz in windows, so it seems that higher refresh rates cause distortion, but 60Hz doesn't.

Still there is no distortion even at 85Hz if the monitor is connected directly to the video card without voodoo 2 card in between.

Just strange...