Here is a recipe on how to combine two or more figures into one
(same effect as subfigure in latex):

convert -background white Vehicle0_hier1.png  Vehicle0_hier2.png +append tmp.png

Use +append for stacking left to right, -append for top to bottom.
The positioning of the figures can be controlled by -gravity.

More general tool: montage

montage file1.png file2.png ... file4.png -geometry +2+2  result.png
montage file1.png file2.png -tile x1 result.png
montage file1.png file2.png -tile 1x result.png

See
http://www.imagemagick.org/Usage/montage/#montage
for borders, frames and shadows.
