Adjust the spacing parameter in the <ideogram> block between the first and last ideograms. Make it as large as 1/2 of the circle - make it the same size as the total size of all ideograms, to divide the figure into halves.
I've attached examples that show how to do this with one ideogram, or multiple ideograms. The ideogram.conf contains the relevant spacing parameters.
If you want the ideograms that start at a different position in the circle, adjust angle_offset in the <image> block, which is -90 by default.
<image>
<<include etc/image.conf>>
# override default imported from file above
angle_offset* = 0
</image>