Well, you could use the grouping tool to spawn props from the one-sided portions, reverse the normals of those props, and combine the props with the original geometry. Not sure how well that would work out.... Or you might be able to use Objaction Twoface, which is available in the FreeStuff, somehow.

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Cage can be an opinionated jerk who posts without thinking.  He apologizes for this.  He's honestly not trying to be a turkeyhead.

Cage had some freebies, compatible with Poser 11 and below.  His Python scripts were saved at archive.org, along with the rest of the Morphography site, where they were hosted.