Combining two video fields into one (to create an interlaced video)
Use

ffmpeg -i top.avi -i bottom.avi -filter_complex vstack,il=l=i:c=i -c:v ffv1 il.avi

See docs for http://www.ffmpeg.org/ffmpeg-filters.html#vstack and http://www.ffmpeg.org/ffmpeg-filters.html#il