Convert Vertically Stacked video to horizontally stacked via FFMpeg
You would crop the full width and half-height. Then use hstack to join them.

ffmpeg -i input.mp4 \
-filter_complex "[0:v]crop=iw:ih/2:0:0 [left]; \
[0:v]crop=iw:ih/2:0:ih/2[right]; \
[left][right]hstack[outv]" \
-map "[outv]" -map 0:a -c:a copy output_3dv.mp4