Glass sorting issues

I've been trying to create a glass material, or arrangement of different materials that will allow me to view multiple glass surfaces through each other with correct sorting.

I initially tried making several one-sided glass materials, all with vfp enabled but with different sort values in the hope that I could create a "window pane" by butting up two surfaces back-to-back, each with different sort values. In the case of a two-window setup, this window pane was duplicated, flipped and placed some distance away. The idea was that I would be able to view one window correctly through the other because the far window would always be showing a surface with a further sort value than the one facing me on the near window, regardless of which side of the window pane setup I was on.

Now this didn't work and with the help of Black Dog, I found that the problem was due to the vfp enabled surfaces stopping any other vfp surfaces behind them being drawn.

I can create an arrangement that does work, by having one of the materials vfp enabled and the other not. Now the funky thing is that the window surfaces always seem to sort correctly regardless of what sort values I set in my one-sided glass material as long as i can guarantee I can never be in a location where I'm looking through more than one vfp-enabled glass surface.

Why does this happen, does the additive alphablend of the glass surface make the arrangement always look the same regardless of whether the furthest surface is actually being drawn first? 

And I think the answer, Kinn, is that Doom3 is achieving sentience! 

I read this, and I think "I thought mapping help was questions like 'how do I spawn in a monster when a door opens?'..." I obviously live in a simpler age =)