I've found out that you can use ffdshow to directly use the avisynth script to cut away the lower screen (in other words, you are recording a single screen because the avisynth script is being used as you record your video).
Here is how I did it.
Use 'ffdshow video codec' (you need to download & install it)
Configure it:
H.264, quality: 100
On the left side under 'input', check image processing and configure it:
On the left side, check 'avisynth' and paste this into its field
Crop(0,0,0,-192)
This seems to be less of a hassle than using Virtualdub but I don't know how reliable that is. The screen seems to be mirrored or rotated and I haven't succeeded in rotating it back using
Rotate(180.0)
Also, sound might be out of sync... I can't really tell, it needs more testing. This would be the easiest solution to straightly record a single screen, but I strongly recommend using the Avisynth script in Virtualdub
after the initial encode.