It is possible to produce 3D movies in AviSynth with the 3D Toolbox.
For example, to combine left.avi and right.avi into a 3D video suitable for YouTube, you would enter the following commands:
leftvideo = AviSource("left.avi")
rightvideo = AviSource("right.avi")
Yt3D(leftvideo, rightvideo)...