Okay if the images remain about the same size and you just want to divide them down the middel use
-crop 50%x100% +repage
A better method is being looked at.
If as you say you wnt to divide the image by looking for columns of a single color, I suggest you look at the script http://www.imagemagick.org/Usage/scripts/divide_vert
The script is writte to divide the image based on rows of constant color, but can be easilly modified to do it by columns instead (or rotate the image 90 degrees
It will seperate the image into either 5 images end_white, first_part, middle_white, second_part, and end_white, BUT a flag can be given to ask it to just not output the areas of constant color, giving you just two images.
You could also modify the script (mail me your changes) to locate the middle section as well.
This is actually known as a segmentation issue. As such by converting your image into a mask of background and non-background areas you could also divide the image using the scripthttp://www.imagemagick.org/Usage/scripts/segment_image
When you have separated the mask areas, you can use them to mask the original image into separate parts.
This however should not be used directly as it will also separate the red 'spot' from the black area, unless the script is modified to only look for non-white pixels segments.
I would love to see both of these scripts converted into a various segmentation methods built into the IM library itself.